How do you make it a 'natural reflex'? Practice. One very good way of doing so is to get a Protoss partner 1-2 leagues higher than you (for example, if you're in Gold league, try to find a Diamond friend to play a custom game with you). This will allow you to play with Protoss few games in a row (otherwise, in general only 1 game from 3 will be against Protoss.
The second thing is to try to figure out a build that works well for you and practice it in each game. I see two good choices:
4 Barracks opening (1 Rax Marauder (vs. Protoss)). Here is its description with some of my modifications. 9(10) supply - scout then, 11(12) Rax, 11(12) Refinery. Once the Barracks is completed, make an Orbital Command. From 1st Rax, if you have 25 gas, build a tech lab (this is very good to have). If you don't, train one marine, then make a tech lab. When the 2nd Rax is ready, you should see if the Protoss player built a cybernetic core. If so, build a tech lab for each Barracks and research concussive shell. Build mass marauders. Using this build you should have enough army to hold off/push forward with attackd. It would be reasonable after your 2nd or 3rd Rax to build a Ghost Academy. Build one or two ghosts, no more. Two should be enough.
1-1-1 opening (1Rax 1Fact 1Port). I can't say a lot here about this build from my own experience, but it will be more reasonable to use it on big maps or in those cases when you want to counter an opponent's Zealot push (be sure to have enough of an army to protect your wall though). Here you will want to get a quick Raven and use him to block Stalkers' shouts for a while. As a result, you will kill their army without significant losses.
Chose one of these choices (if you are not platinum/diamond I would suggest to practice the first, it will improve your micro significantly too) and try to use this build 4-6 times in order. Afterwards, look at each replay and see what are you doing wrong. In the meantime you will have your 'reflex'.
Also, don't forget to scout, as sometime you will need to change your build depending on your opponent's actions. For example, let's say they made an early Robotics Facility. If you have a lot of marauders that means he's going to follow with Robotics Bay and some Colossus. To accomplish this, Protoss will have invested 900 minerals and 700 gas into getting their 1st Colossus with a big range. Here you have 2 options:
- get nine more marauders and push to kill the Protoss player (or at least damage him significantly), or, if you can't push,
- get a Starport ASAP in order to counter the Colossus.
Another idea to become familiar with Ravens is to practice 'Raven/Banshee' against Zerg. Raven protects banshee from both queens and Spore Colonies and can safely kill Queen/Drones/Extractors. This works especially well if the Zerg player expands early. In this case (from my experience), you are protected from early harassment and have some time for teching up to getting Raven/Banshee (the only precaution: don't forget about making a land army when it's crucial). In this case, usually 1 Raven is enough.
One more thought: instead of an early Raven, you can invest in Banshee Cloaking shield. Often it work better as you can get two cloaked banshees instead of banshee+raven.