(If you haven't tried this) Why dont you have your team position themselves to cover ways of the opposition getting around and through you. For example: lets just say these are the circumstances....

Level - Blood Gulch Weapons - Normal Vehichles - Warthog

Your Base - Red Base

Ok now, before the other team makes any progress in attempting to whoop your ass, get one of your players to take a sniping position up on 'sniper rock/hill' so that he can take down people no matter where they are. Have another player quickly take him in the warthog, drop him off, while at the same time have a player in the gunner seat. This is so you can provide good offense and still have great defence (remembering that the player remaining is still in the base so make sure hes got a shotgun (closed areas) ). Now you can drive down to their base and attempt to steal their flag with a fat ass gunner coverin ya, while their offensive players get taken down by from the greatest sniping location on the map as well as a base defender wielding a heavy impact weapon designed for spaces like bases :D.....hope this helped