Hint: You do not need a rocket to do this, but it helps.
Go the three-story building in the center of backot. Climb the ladder to get to the third story of this building.
Once climbed, turn to your right, walk forward, and press A to get up onto the railing. Now turn right again. From here, you should back up, and then you should be able to sprint jump onto the top of a telephone-pole in front of you.
Position yourself in the center of the telephone-pole. Once positioned, take out your rocket and look straight down. (This next part will be hard for a while, but with some practice, trust me, you'll get it.)
Without ever pressing LT to zoom in, press RT to shoot your rocket, and then immediately sprint and jump off the end of the telephone-pole torward the right side of the palm tree ahead of you. (During this jump, never look up.)
If done in correct succesion with good timing, you should land in the top of the palm tree. ( This itself is a pretty cool sniper spot).
From here turn a little bit to your left and you can sprint jump on to the top of the turret building. From here you can just walk to the back of the building and fall out of the map. (Please, this will take some practice, and if you can't get it, friend me and I'll help you out when I'm online.)
Also, you do not HAVE to use a rocket to jump into the tree, but without it, I find the jump next to impossible.
[more cheats for Call of Duty 4 here]