VanillaBean- Thanks! Ok, here's how I did it. Get on him and take him to one place every time away from distractions. Choose a voice cue. Mine is Up!Up!Up! really fast :). When you're ready, use the voice cue you've chosen and pull straight up with both reins in one hand (Don't pull hard, just pressure) and sqeeze with both legs. Hang onto some mane with your free hand. He's gonna try to figure out what you want so keep doing it, but don't pull harder or kick harder. The second either of his legs leave the ground even just a tiny, tiny bit, praise, praise, praise, praise, praise. Then leave him alone for a couple seconds. Then do it again. The key is to praise for even the slightest effort. Soon he'll get good enough and you can ask for a bigger and bigger rear. I would recommend doing this bareback if you're comfortable or with a really light saddle.
Of course they're is the safety issue with this trick. I strongly reccomend that you read up on that. If you teach it to some horses, they think that they can rear whenever they want or if you want them to do something that they don't want to do. That can be hard to train out of them but it can be done.