Defensive Strategy:
If you have a team of 5 or 10 members, designate some people to defend the bases you already have. All the time in multiplayer games, you'd be close to winning, and the other side gets a base all the way on the other side of the map wayyy behind you. Sometimes they even make a come back and win that way. If you have have 1 or 2 persons designated to each base, you'll help to prevent them taking over from the rear : P
Also, if you haven't already, make sure to have all the bases in either the north, south, east, or west of the map so you aren't fighting a battle on both sides of you.
Attack Strategy:
Pretty much taking advantage of a person who didn't follow the defensive strategy above. In other words, get a few of your troops, preferabbly 2 on each side of the map, and head toward the enemys base wayyyyyyy in the back, and then attack em from behind.
Here's some random strategies:
If you're attacking a walker, or any type of vehicle up high (except for planes of course), walk under them so they can't shoot you, and then use concussion grenades (they stick to the metal). You can also use thermal detonators, but since they dont stick, try to aim for the top of their head. Lately i've been throwing the grenades so they land on top of their head, so it stays and doesn't fall off :) Same goes for misslez, get under them, wait for them to walk away a little, and then shoot em with missle. If you don't wait a sec for em to walk a lil far, u'll blow urself up (i've some so PLENTY of times lol). Mines might be a good strategy too, but haven't tried it to see if it is effective or does much damage.
If you're a sniper, or someone with a relatively small amount of ammo, and you're on a relatively open map, stay next to the repair machine and the ammo dispenser. This works better if the enemy is farther away and not right on top of you.
If you have a few extra troops, get one into a walker, and have other troops escort it (so then you can attack people who are behind it that might shoot missles at it).
Most of these are basic, but some people might not of thought of em.