I can add a piece of info in this topic..
1. Be a teamplayer, by that i mean, repair tanks, repair buildings, donate to players that just joined the game (other story if you have no Refinery)
2. Make sure ur buildings are safe, if u think ur buildings are not safe enough, mine the doors/tunnels/entrance to prevent the enemy sneaking inside ur base and sabotage your game.
3. Join rushes, the more the firepower, the more the chance of succes.
4. If you hear EVA say like 'Warning, GDI Weapons Factory Under Attack' , you can press the 'K' button to see the health bar of your teams' buildings, as well as your enemies' if an building is underattack, make sure u or one of your teammates repair it, or otherwise, well you know..
5. Low on money? No refinery? grab a soldier (automatic rifle) and shoot the enemy vehicles, if you do this, you will get like 200-300 cash in like 1-2 minutes, BEAR IN MIND, if the vehicle is under 50% health, you wont get cash that fast, so shoot the vehicle if it has a >GREEN< health bar.
so far. these are my tips, any others have some more?
Portal 
