I shave everything, balls, crack, all that. I have been for about 4 years now. And I'm a pretty freakin' hairy guy, so it's quite a chore for me. A few things I've found that help:
Use beard/mustache trimmers first. They're smaller than regular clippers, which makes them easier to control and easier to get into small areas. Careful on the balls - even with a guard on the trimmers, I've nicked myself there pretty good. After trimming it all up:
Wet the area, then apply shaving cream, and let the shaving cream stay on for about 5 minutes before you shave. Softens the hair. After the five minutes, put on a little more shaving cream, just for good measure.
While shaving with the razor, NEVER go against the grain. Bad things, man, bad things. Also, never shave the actual asshole. Too easy to get infected. The trimmers work just fine for that area.
As far as where to stop shaving, I still haven't come up with anything concrete. Like I said, I'm a pretty hairy guy, so my leg hair, pubic hair, ass hair, and stomach hair all merge together, there really isn't a starting or stopping point for any of it. When I first started doing it, I tried shaving all the stomach hair, too. That worked OK, but it looks kinda goofy when you go to the beach or something with one or two days' growth on your belly. Now, I stop just below the belt line. On the way down, I stop right at the little crease where the groin meets the thigh.
Sorry if I was a little too graphic for some folks' tastes, but I'm just trying to help some people out. I wish someone would have told me this kinda stuff when I first started doing it.