This was probably already mentioned, but for the few who don't know about it yet... a little while after you gain access to Trevor, there's a Stranger mission near Trevor's trailer in which Cletus wants you to go shoot stuff with him. Start it, then walk away. You can get into the military base, airport, prison, etc, with no stars. If you've progressed far enough to gain access to the Sandy Shores hangar/helipad, you can walk right into the military base like you own it (haha), take anything you want, bring it back and store it. Leave that side-mission unfinished to gain uninhibited access to these areas any time during the storyline.
First, might wanna read up on some kinda glitch involving storing the fighter jet at the Sandy Shores hangar. I didn't really understand it, but I haven't experienced it either. Might even be fixed by now.