It is difficult to walk into Fort Zancudo because of the immediate 4-star Wanted level the Army will throw at you. It is even harder to obtain the Cargobob or one of the Army tanks. To enter Fort Zancudo without a single star level, you just need a friend and this cheat sequence:
Go to the Marsh area outside Fort Zancudo and ask your friend to kill your characted so you can start a One on One Deathmatch with them. Steal the vehicles from Fort Zancudo within five minutes after the match begins and of course, make sure you don't get caught. If you get to do this, your wanted level will not rise at all.
Be sure to be completely out of the military base before the Deathmatch is over otherwise your wanted level will still rise to four. If you want to end the Deathmatch early after you have stolen everything you want, just go far away from your friend's characted and the match will eventually end.
Aside from the glitch that allows you to repeatedly resell your car to the LSC (Los Santos Customs) and make you a ton of cash quickly is a newly discovered, legitimate way allows you to earn money by trading of cars. The player should tread carefully when it comes to glitches because the devs usually fix it and sometimes, players also get penalized for performing cheats. This is how you can do the glitch: The car's value should be around $15-$20 grand and should not be valued above $50 because LSC will not buy it from you. You can navigate the sell option to sell your car and immediately afterwards, hit the Start button, navigate to the Xbox Games store tab and hit A. As it loads, press B to back out so the car would still be there for reselling and you just keep earning cash.
This is how you do it legitimately: simply offer your low-cost cars to the LSC and make a quick buck. LSC accepts a car for every 48 minutes (24 hours of in-game time). This cheat only works on Xbox 360 games, though. There are specific cars that give more money through LSC. See the list of cars and its respective LSC payouts here: