Often one team can't move out of their safezone very well or people lose a lot of money doing so, when another team dominates (mostly on vehicle servers).
When one team is X(maybe 5?) points behind another team, give them some form of advantage.
Cheaper transport vehicles or more free vehicles in spawn.
Also if these vehicles are destroyed by the enemy, they receive less or no rewards for it (still normal kill reward for the people inside).
Also make them despawn faster than normal, bought vehicles.
This can help people break out of a spawn camp and at the same time doesn't give the spawn campers even more money due to even more transport vehicles getting killed.
More suggestions are welcomed as comments on this post.