This is for the German Version of Saints Row 2 (SAINTS ROW 2 BRD), which is not uncut.. some of the activities and story parts are missing! so you may check the other solutions writen for the original as well..
There are just a few helpful things to know.
1. Learn how to switch weapons fast!
2. Don't use your best weapons like Bazooka or Assault Rifle in the beginning, save the ammo for later deals in that level! For each level of the activity you get new ammo...
3. Use your unlimited ammo (most cases it'll be for your pistol)
4. Always!!!! Aim at the driver
5. Never!!!! shoot the car right behind with your rocket launcher you'll need some distance
6. Often switch cars and recruit some backup when you stop and have the chance..