I watched Full Metal Jacket about 16 times before I left for boot camp, and it actually made me feel better about things, but then I am nothing if not a little crazy. Here are my tips:
1. Your going to reach a point where your down, and you feel like quitting. Everyone, and I do mean everyone, does. There is no simple answer to this feeling other than to point out that those three months of boot camp will help define your life forever. You must man up and just get through them. In short, the bad days will pass.
2. Learn to never expect anything from the Corps, in boot camp or otherwise. You are guaranteed food, a place to sleep, and the occasional kick in the ass. It is up to you to be a professional, and professionals make their own joy. I never had any expectations going in, and I had a blast. The guys that had their careers planned out 15 years in advance always seemed to be the guys that turned into s-birds. Just saying.
3. Understand that you are going to come out a different man than you went in, and as a result you will have certain expectations on you the rest of your life. Some guys may poo-poo the concepts of Esprit de Corps and Brotherhood, but I don't, and believe me, there are many thousands of other Marines that don't. Trust me, for every drop of sweat and blood that comes out of you, there are literally thousands of us that are rooting you on. I am going to be a college graduate in about three months, and I have done a lot of interesting things in my post-Marine Corps life, but being a Marine is still the thing that I am most proud of, and it always will be.
4. Remember that boot camp is vastly different from the fleet. Take it all seriously, as that is your responsibility as a professional, but also keep in mind that the fleet is a different and substantially more fun place to be than boot camp.
5. Finally, and this sounds cheesy but I swear to god I mean every last word of it, remember the Marines that have went before you. For every pushup, side-straddle hop and mile you run, there have been hundreds of thousands of men before you that did the same thing. Also, keep in mind the Marines that have died for our country. Their sacrifice is so much more meaningful and important than whatever paltry little side pain you feel on a run or whatever. Believe me when I say that you can draw a tremendous amount of strength contemplatinf that. I did.
Good luck. Work hard and be a professional.
Oh, and by the way, you will learn to HATE airplanes at San Diego. Hate them. Passionately. Every day you will have to try and listen to drill commands with a 747 taking off (impossible at first, but you learn to do it), and every day you will see bright shiny tubes transporting hundreds and hundreds of people to far more enjoyable destinations than the one you are at. Trust me, when your in the pit getting pushed and you see "Air Hawaii" take off, you will not like it. Not at all.