Its pretty simple. Jack a big rig. Drive around the outside of the base until you reach the control tower. At the control tower on the outside of the base there's a concrete wall. Park the big rig next to it and get out. Scale the big rig and then the fence. And TADA you're inside with no wanted level!
I just tried this and got an instant 4 star wanted level as soon as I climbed onto the concrete block.
I've made it in twice now without any stars. Once during the day and once at night. The night time one went a lot smoother once inside. I was able to make it damn near all the way to the jet that sits out next to the runway without being spotted. Just be mindful of people, lights, and the cameras on the walls.
It does work, but you will eventually get wanted stars, but the time t takes to get the stars depends on how long you can avoid being detected.
This is a good way to steal one of the jets parked on the runway. Hop the wall, sneak in and run like hell. Because there's a couple of soldiers near the fire trucks, you'll get spotted, but you usually can make it all the way to the jet first.
There's a Youtube video of someone doing it. I'll post the link if I can find it again.