1. first thing, if anything. ASAP start droping prox mines, in maps like under, I find this to be the first thing you should always do2. get a good offensive AND defensive
3. don't waste all your money on vehicles (exp. if your teammates are also wasting their money) and just rushing a heavily gaurded base. SO many times I see my team do that, then when they actually attack, we have no money to buy vehicles to defend ourselfs.
4. whenever possible, try to kill any support engineers/hotwires/techs.
5. for nod, kill GDI's harvestor early, I found this to be very effective since gdi's vehicles cost alot. GDI can do this as well, but it doesn't show as good of results, since they're stuff is cheaper. Also, protect your own if you can. If the enemies harvestor is stuck, just leave it.
6. keep moving. try if you can, to never stand still, when in vehicles and when your not in them.
7. cover all interences to your base, exp. in maps with no defense. Doing this will help keep pesky engineers/techs/hotwires from destroying your vehicles
8. heal whenever possible, and as fast as you can. Try to keep your tank alive as much as possible, and to be safe. Even if it's not completely healed, get in and drive away. This is good for gdi if invisible black hand should be near. Also, try to do this in your base as much as possible, much easier.
9. During battles inside your buildings, if your getting low on health, hit a Perchase terminal and quickly hit 5. It's cheap, but hey. It gets the job done. Very Very Effective this is. Good for killing pesky infantry.
10. Keep your distance. Always try to stay away from oncoming tanks and such, and try to aim infront of them so your shots more so hitting them.
11. Dodge fire, jumping and ducking are good ways (just hit duck and let off) to avoid critical damage from sniper fire. Always pay attention to the surroundings.
12. low class snipers, snipe humvees/buggies and artilary/MRLS as well as infantry. I found the sniper is pretty decent agenist those vehicles! I don't use 1000 cred snipers, so I can't say anything bout them
These are my words of advice. I found these to be very effective. Enjoy