Break from the balk line about 2-3 inches from either rail.
Hit the 2nd ball in the rack head-on as hard as you can.
From my experience you've got roughly a 50% chance (perhaps greater) of making a ball.
If you're playing lower-ranked players they're unlikely to run out on you if you don't make one, so I think the odds in that case make it worth playing this shot.
Holy crap! As hard as I can? I'd say I have about a 98% chance of scooping up whitey from the floor about 30 feet away. I want to see someone, anyone, break for the 2nd ball on a bar box as hard as *I* can.
IMHO, there are several qualifying questions for the OP. First, 9 ball or 8 ball. 2nd, does 8 on the break win?
So, for 9 ball: I break from 1/2 way between the rail and the first diamond. Cue ball right on the headstring. Hit the cueball as close to center as possible. Also, hit the head ball as square as possible. I hit this break about 60% power. I'd say around 18 mph. I have been doing this at Valley Forge all week. I think I make a ball on the break about 90% of the time. Often 2, 3, or 4 drop. Granted, these are fast tables with near perfect racks (they are using the Accurack, a pretty cool device that offers a very consistent and repeatable rack). I have had similar success on other Diamond bar boxes with standard racks (racked well), and slightly reduced success on Valley 7 footers. I agree with Neil that soft breaking can work fantastic on a perfect rack, but is pretty dangerous and unproductive on less than ideal racks.
Edit: I should also note that you can have similar success breaking from the rail the same way. I read some article on the 9 ball break by Blackjack. It was excellent. Follow those principles and back off a bit on the speed. You'll be fine.
For 8 ball: I break from dead center on the headstring. I hit the head ball as square as possible and pretty hard (23-26 mph). I find I make a lot of balls. Under ideal racking conditions, you can back the power down a bit, say 20-22 mph. If you are not making balls that way, or especially if the head ball is not freezing well to the balls behind it, you can try the 2nd ball break. I break from 1/2 way between the side rail and the first diamond, right on the headstring. I hit maximum low, and aim for a half ball hit on the 2nd ball. I break medium hard, say 18-20 mph, maybe even less. This break relies on a lot of accuracy to avoid serious catastrphies, like flying off the table, accidentally catching a slice of the head ball and scratching, and worst of all making the 8 and scratching. However the likelihood of making a ball is great. My main reason for not using this break much is that you give up a lot of control of the cueball. It is far more difficult to predict the final location after the break, because the cueball will not only be moving more, but will be doing so in the area of the table where the most balls will be moving. Expect a lot of collisions with the cueball.
I think the most important advice for breaking on the bar table is this: don't try to hit the rack hard. Be smooth and easy. You can accomplish a lot with a little. Its all about accuracy, and since you are so much closer to the head ball of the rack, this can really help with that accuracy, so really take advantage of it.
Hope this helps,
KMRUNOUT