I don't agree with surrendering a game like a lot of the people in LoL do. I found that a lot of games can be won despite being in a sticky situations by playing defensively and farming while waiting for an opportunity.
Other than that, are there any other effective strategies to winning a game where you are at a massive disadvantage?
Farm. Farm. Farm. Farm.
The more farm you get, the more on level with your opponents you can be. Stay with your teammates and group up to take towers, buffs, dragon and or baron. And always keep your head up and never give up.
As I've learned playing over the past 3 years, its to always have a positive attitude, because you will never know when a teamfight will turn in your favor and lead to a possible comeback. It's happened to me many a time. I promise you that.
Sometimes forcing a fight at Baron can allow you to come on top. Make sure your team is fully prepared to fight and sometimes the enemy team may come in out of position, giving your team an advantage.
Try and catch people in the jungle with your team. If you can kill one person you will have a huge team fight advantage, so if at least one is down go baron. If you can catch just 1 person it can be enough to win.
If you're losing, then you're probably down on farm. You need to make getting gold a priority in order to catch up. You need to prioritize objectives such as baron and dragon, since those can really level the playing field in a team fight. If you can win a team fight, make sure to capitalize on it by getting baron or a turret or two.
If you're really backed up in your base, you can actually come back to win because you can farm quite easily while the opposing team is severely denied creeps. Provided you don't lose the nexus, you can actually farm you way back in the game by killing those super minions.
When you are in a losing game make sure to stay on the Defense and try to catch some of the enemies out of position for a kill. Do not push the enemies' creeps too far alone. It is helpful to have champions with easy escapes like Tryndamere and Twisted Fate to backdoor their turrets and progress. ALWAYS stay as a team (Other than the backdooring people) you just start feeding when you are caught in the jungle alone.
The single biggest thing is to keep your morale up for your team. Make sure you don't make mistakes like get caught and wait for them to make a mistake like get caught out of position. If you have strong initiation, like Ashe arrow, Blitzcrank hook, Ahri charm, those can quickly turn a teamfight if done on the right person. If you believe your team has the needed damage, make sure your team keeps good positioning and understand who are the main targets to focus and role for teamfights. Simple things like everyone just protect and keep enemies away from our Vayne or KogMaw may need to be emphasized.
