Stealth and speed.
First, get a good scout tank - one fast, with high camo value. Personally, I recommend ELC AMX - highest camo value in the whole game, and enough speed to reach extended spot and hide before enemy shows up.
Give it the camo net and binoculars. Rent camouflage pattern (for credits, 7-day or 30-day). Get a good crew (ELC is only 2 people, so 400 gold isn't that much if you don't want to grind). Choose camouflage skill for them. (once you reach 100%+41% next perk, reset and exchange it for 6th sense for captain, it is even more useful.) It is active for moment one, and first 60% or so advance really fast. These things really add up.
Next: Approaching the spot. There are two methods: Fast and cautions. There's also third, far less efficient: Spot&run
- Fast, is when you drive madly to the chosen spotting point at maximum speed before enemy gets in positions to see you hiding. When they arrive you're hidden already. Notable maps: Malinovka (Encounter), El-Halluf, Erlenberg (defense), Highway, Murovanka, Siegfried Line (defense)
- Cautious: Approach from behind cover, advance as second behind some lemming who distracts the enemies with a suicide attack, drive far around, etc. Alternatively drive behind a solid cover that is adjacent to some bushes and sit behind it for at least a minute, until you're forgotten. Notable maps: Steppes, Mines, Komaryn [and fall back as enemies push!] If you advance through a forest or bushes area, advance slowly from bush to bush, hunker down and wait for binocs to kick in. If the route is clear, advance to next bush. If you spot an enemy, wait for the team to dispose of them before advancing more.
- Spot&Run: Some maps simply don't provide any good spotting points or the battle is too chaotic or too dynamic to guarantee staying in one place. Spot and hide behind solid cover before enemy can hit you, or just give up scouting and flank for kill instead. Notable maps: Lakeville, city maps.
Note: you don't need to highlight the whole enemy team. If you provide 1-2 good spots, you're doing your work right and it's the team's turn to kill them. Consider changing position once they are dead (but more often than not there are more - hidden. You'll highlight them when they shoot and become visible to you, which is still your job being done right - note trying to advance on a TD from the new line is totally suicidal - with you on the move, without the camo net active, a TD like Rhm. Borsig-Waffentrager will have you spotted long before you get in range to spot them. So, if you have a good hiding spot, think twice before changing it.
Next: bushes. ALWAYS hide in/behind bushes. They more than double your camouflage values and hide you before the camo net activates. Press RMB to prevent turning around and look at your tank from enemy-facing side verifying all of your checkpoints are hidden. If the bush is big enough, hide whole in it, that way you won't be spotted even if the enemies circle you. Use "Lock tracks" (X) to prevent turning when looking around. Sit motionless and just let others do their work.
While as Light you get the same camo bonus while moving, Camo Net gives you a considerable advantage. Remember: spotting isn't instant. Spot checks are done periodically depending on distance between you and the enemy. The bush and your bonuses may keep you hidden through first 3-4 spot checks by enemy, and then camo net kicks in utterly thwarting their chance to spot you - as long as you remain motionless.
Get yourself a cup of coffee and enjoy the Ka-ching! of credits and XP falling into your account with your team destroying targets. You may request fire on enemies that tend to approach too close or suggest other targets but generally don't move and DON'T SHOT. There's one situation when you are entitled to shot: You just end scouting, be it due to imminent death of your last enemy, or imminent discovery of your position.
Now this would be too easy if it was so fault-proof. If you're spotted while near cover, you may retreat and hope they forget. If you are spotted in the field, run for your life, which is unlikely to be long now.
Now there are two ways you could be spotted:
You were not cautious enough. You didn't reach cover in time, you moved, a piece of your tank was sticking out, or even (Gasp!) you shot your gun! Usually you learn about this by dying. Which is very fast and there's usually nothing you can do. Run for your life if you can, which you usually can't.
An enemy approached you within their spot distance. Which is roughly 50m with all your bonuses activated - very close, but that happens fairly often. Now THIS is fairly survivable. First, draw friendly fire to anyone approaching. Next, if that doesn't help, and they do move within your gun arc, do shot them AND RUN. The enemy will often fail to hit you if you move fast enough and if you kill or damage the one who was nearest they won't have a chance to respond. Nevertheless, just run before it's too late.
Note on some maps this is quite likely and you should simply accept the enemy scout WILL get you within their spotting range. For example, the forest in Murovanka or the bushes at in the center the northern side of the Swamp. In these situations it's often prudent to park your tank facing AWAY from the enemy to make your rapid retreat faster.
After you've been spotted, the nice, cozy minutes of silent spotting are over. You stand little chance of revisiting the good spot point this late into the game, even assuming you're still alive. Work spot&run, hunt for support fire opportunities, even suicide-scout at stalemate locations.
Let me cover a bit on suicide scouting. It's a valid technique in some circumstances, but the hardest to do right, the easiest to botch, and the most luck-dependent. As result, the efficiency of suicide scouts is dreadful and the team very rarely profits from that at all - a dead scout is of no use for the team.
This is seen very often in early tiers, so some players get the wrong idea that this is what all scouting is about. They will scoff at the scout correctly placed to observe a major part of field, but staying motionless, or them, themselves will drive ahead and die trying to pretend they did what had to be done - scouting.
Some dos and donts of suicide-scouting.
- Don't suicide-scout if you have other options. You'll likely end up dead, and dead you're useless.
- Wait for your team to get in position. I can't count the times when I placed my arty, switched to sniping mode and found two of our scouts dead already. That's completely useless. That doesn't mean "don't drive out early". Just drive near the enemy lines, fall behind some obstacle, wait for team mates to spread out, aim their guns, then go.
- Don't scout outside your team's draw range. That may give a bit of tactical advantage of knowing enemy layout and earn the arty a shot or two, but otherwise it's mostly useless.
- Move fast, turn often and try to be unpredictable. Yesterday I took out ELC AMX with a KV-2 from half the map away, as he was making a third circle in the same place. That would be impossible if he didn't become predictable.
- DO suicide-scout in stalemate situations. If the enemy formed a coherent defense your team has a hard-time breaching, a good suicide scout is what your team needs to turn the tides. Enemy's turrets turn, tanks move, they stop paying attention to the attacking side, they just try to hunt you down. This gives your team enough tactical advantage to win the battle.
- If you do deep-scouting, destroy the arty or at least damage it. You'll often be out of your team's draw range and the arty will likely start moving when spotted, besides, your arty will take some time to zero in. Better get these kills if you can.
- Since your range will be likely longer than with stealth-scouting, good radio in your tank is a must. There are few more frustrating things than a scout that vanishes from radio range.
...nevertheless, if you can scout stealthily, don't suicide-scout. Alive, you're more useful.
Concerning the surprise hailstorm: it might be the artillery.
I'm playing almost strictly artillery and it has a very specific playstyle:
- the radio is the holy spirit of the artillery. They almost never depend on their own spotting ability, hidden far away. They observe the map like vultures and focus even on tanks other just pass by, spotted automatically, or ones their team choose to avoid engaging.
- The reload times are long. Sometimes 20s, sometimes more.
- Aiming times are long too. Good aim may take another 10s or more.
- They also observe the terrain in sniper mode for hints of enemy activity - broken environments, damages.
As result: if you remain immobile and get spotted even for a few seconds - even in passing and completely ignored by a passing enemy tank, the artillery will focus on you. The tank that managed to spot you may be far away by now, and the artillery is waiting for their sniper mode to zero on your position and then launch their barrage blindly at your position when reloading and zeroing in is completed. If you are boxed in in some easily destroyable environment they are easily able to tell you didn't move out because the environment is intact. If there is no crater left after their shell hitting, it means it hit you and if you don't move there is still sense to hit that spot again. So, getting spotted even briefly while immobile will expose you to artillery fire which may come much later.
Addressing xBeats_GhoStZz' comment:
ELC AMX is not a good SUICIDE scout. Yes, if you drive behind enemy lines, you may not be able to broadcast enemy positions to your allies. Then, if you drive behind the enemy lines, no matter what light tank you used, you rarely live more than a minute too. That means at most 1-2 arty shots, and your team usually too far to lead accurate fire to whatever you spotted anyway.
Nobody but arty profits directly from enemies spotted further than 550m* from them - enemies outside their draw distance don't get highlighted in direct or sniper view, and hitting them blindly is nearly impossible. SPGs have wonderful radios that will get the ELC within range no matter how far it goes. The others simply won't profit from it scouting beyond combined its+theirs radio range as it will significantly exceed their draw distance so they can't hit the spotted enemies anyway. If you scout that far, you're doing it wrong. The only point could be hunting enemy SPGs but that can be done without support of others anyway.
ELC is a perfectly good scout for sitting halfway between two sides exchanging blows at edges of their draw range - dead center of Malinovka field, 2/3 through sides in Komaryn, forest in Muranovka, cliff-sides in Cliffs, the mine in Mines, the middle of the valley in Lakeville. Besides, it plays up to IX Tier battles. Most tanks by then have radios good enough that they really don't need to depend on the ELC's radio anyway.
* 550m in Manhattan Taxi metrics = within 1100x1100 square with you in the middle and sides parallel to map grid. That means a tank 750m exactly north-west from you is visible, while one 560m away due north is not.