Man, three people PM'd me for 8 Ball breaking advice. I'll cut and paste what I responded to them with ...
-I break from the right side of the table, about one ball's width from the rail, at the second diamond (basically as far forward as I can go while still being in the kitchen).
-From here, I like to hit the second ball in the rack - the one immediately behind the head ball. The trick is to hit it as FULL as you can, with a good amount of back spin, about 3/4 power, and with a LEVEL CUE. Having a level cue is the most important part, otherwise that cue ball is going off the table every time. If you hit too low on the object ball, the cue ball is going straight into the corner pocket, so really practice hitting the full ball.
-The backspin will usually cause the cue ball to go to the rail after initial contact, and then come back at the rack where any clusters remain. Sometimes this leads to a lot of the balls ending up on the left side of the table, but it's usually pretty manageable.
-Everyone has a tendency to want to hit the ball as hard as they can, whether they are hitting the head ball or not. On a barbox, that often leads to the balls spreading, but then coming right back into each other and clumping up again. So, even if you decide you're more comfortable hitting the head ball and not the second ball, try to find a comfortable speed to hit the ball at around 3/4 of your total power. Accuracy, as with any break shot, is important on a barbox. The less off-center you hit, the better the spread. The better the spread, the less traffic (which is often the major problem in barbox pool), and the less traffic you have to deal with, the more you run out
-Like I said, if you don't want to hit that second ball like I do, then I'd recommend breaking from somewhere a little closer to the center of the table. Breaking from the rail just doesn't seem to work for me on a barbox. If you are going to hit the head ball, I'd suggest hitting it as square as you can. A cut break, although successful in 9 ball, just doesn't work for 8 ball.