I don't know how you would practice this, unless you're blessed with a very patient (and very loving) female best friend. =)
There are two main kinds of bras you run across, one has what I call the "X-type" lock, where the two clasps hook into another like an X and then snap, the more common type simply uses four hooks arranged in a rectangle to stay in place.

How to approach this

It's needless to say that this should only be done with bras that close behind her back. Get behind her, and after a massage or at an opportune moment, seize the lock with your chompers. If it's an X-hook, you have it fairly easy. Simply pull it back about half an inch and cheat using your pinkie to actually pop the lock open. (You're behind her back, aren't you ;P.) Then use your tongue to slide the clasps apart, Don Juan style.
More likely though, you'll be dealing with those pesky, pesky hooks. This is where you sort of need to practice. If the bra is elastic enough (slide your hand under the strap casually beforehand to test the waters,) you can grasp the whole mechanism with your front teeth. In a quick motion, you flip the whole shebang.
It is imperative that you move your head back while advancing your jaw and flipping the clasp, or you'll shove your jaw in her back. Quickly move your teeth closer together while flipping it; pull them apart (your jaw back), unhooking the lock to her treasure chest when it is inversed.
"But what if it's too tight?" In that case you'll be better off to just use your thumbs, and in one motion brushing the clasps together, unhooking them.

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