A very easy way is go to the cliffs north of the water plant on the north side of the base. The cliff above the farthest west pool works best. Wait until you see tanks driving around, you may have to run up and down the side of the mountain a bit. When you do see a tank, shoot it with a sniper rifle 1 or 2 times. Try not to get more than 2 stars. Once you get the tanks attention, get to the bottom of the cliffs, this will allow them out of the gate. Run back up the hill and hide from them, don't go too far or the cops come. Once you lose your wanted level run down to the bottom and jack a tank.
This, if you are far enough from the gate they will just drive through, make sure you look directly at the tank once your wanted level is gone because it will vanish instantly.