It REALLY changes for me based on the game.
A game like Americas Army, I practice every map, and since every map has objectives, my clan and I set up ways to guard the objective, and good tactics for taking it. Since most of us are military vets we all apply real tactics to it (Binding overwatch, formations, sniper/scout teams, etc)
A game like AmrA: Armed Assault, We do a camapign I scripted, and to train for that I made training courses kind of like what the army does (so that way we dont die in the game and have to restart hte mission I put us up against wall targets and empty tanks, I know, i get a little to real and hardcore at times).
But on games like Call of Duty 4 / World at War, I just keep playing. On specific map modes and such I will set up a role. Such like on CTF on WaW I will use the browning M1919 with a bipod and set up a defense position with my sniper (my clan is hardcore competitive on gamebattles). On War mode, I will use an SMG with double grenades. But as for actual training, there is none, I just keep playing, learn the map, learn what works, and apply it come match time.