Do NOT put the u-lock around the frame. Don't buy a u-lock that is wide enough to go around both the rear wheel and the frame. Many dumber crooks use prying tools to attempt to break u-locks. A u-lock wide enough to go around both the rear wheel and the frame gives the crook plenty of room to insert his prying/breaking tool. But, the seat tube begins to bend before the lock begins to bend. So, "Mr. Dumb Crook" ends up destroying your frame.

Use a premium "mini" u-lock. The new Kryptonite Fahgetaboudit mini u-lock (weighing a hefty five pounds) is probably the best of the bunch. A mini u-lock is big enough to get around the rear wheel and a beefy steel post (by pressing the wheel up flush against the steel pole). But, there is not enough empty space to insert effective prying and breaking tools. The worst possible damage to your bike, if the crook applies a massive amount of force, is that you will need to replace a rim...much cheaper than replacing the frame.

You doubt this method works? Just try it out. Lock the rear wheel of your bike, with the lock just an inch behind the seat tube, to a beefy steel pole. Release the quick release, and try to take the wheel. Try to take the frame. Neither of them is going anywhere.

A crook learns very early in his career not to even bother looking at a bike that correctly uses this method. There are waaay to many bikes out there that use cable locks, or have only the front wheel attached to the rack. Those bikes are "free" bikes, and those are the ones a crook will go after.

I suggest the Fahgettaboudit Mini, because it is reasonably easy to locate. In fact, it is currently be sold on E-Bay for $59 with priority mail. Abus makes high quality locks, but they are difficult to find in the USA.

The very best (and most expensive) chain locks (the ones that weigh eight or ten pounds) are as good as the Fahgettaboudit Mini u-lock. But, I don't see any reason to carry that much weight when the a mini u-lock can do the job with half the weight.