This is what i do (normally):
Seal

If i join a game at the beggining, I will leap straight over the jump over and leg it to the fish market. Usually you can get some people not expecting to see you ther

If i join a bit later and we are a few round down I will set up camp, I will put claymores down and hide in the spawn or by the jump over wall.
Weapon of Choice for both tactics: HK5K or M14
Terrorist

Grab a 552sd duck under one of the tents in the center and wait for victims. People will most likely walk straight over you.

Grab a powerful gun maybe an m14 or an m16 and charge the restraunt, get under than bridge before its blown throw a nade through the window and charge in guns blazing.