Just to add to what dpvu said. At the end of a game before you remove your battery be sure to fire it a couple of times in semi to return the spring to it's resting position so as not to have the piston in the half cocked position. Like he said, it will put unnecessary strain on your spring and may weaken it.
Originally Posted by dpvu
Dry firing is bad because without a BB, the piston head slams forward into the gearbox with almost no resistance. This can lead to your mechbox breaking faster (especially if it's a V2). Sometimes you have to do it (such as clearing your gun and testing) but keep it to a minimum. You should pull the trigger all the way to ensure your gun does cycle but if you don't and it locks up just put it into full auto and fire a few BBs and it should be fine. It's not going to kill your gun (may weaken the spring if you left it for a really long time) but there's no reason you shouldn't be pulling the trigger all the way. Rain is fine for AEGs for the most part. Just be sure to keep the battery compartment dry and also clean and dry your AEG well after the game to prevent rust.
There's lots of maintenance guides online.