It's a matter of putting the new piston through several heat-cool cycles, and allowing the new bearings to match the surfaces.
1. Start the engine and warm it up gently, with small throttle blips to keep it from loading up. No high revs! After it is completely warm, shut it off and let it cool completely.
2. Repeat #1.
3. Warm the engine, then ride it gently, using no more than 1/4 throttle, for about 15 minutes. No high revs! Let the engine cool completely.
4. Repeat #3, this time using 1/2 throttle.
5. Repeat #3, going to 3/4 throttle. After 15 minutes, do a couple of full throttle runs through 2 or 3 gears to ensure that rings are properly seated. Let the engine cool completely.
6. Now you are ready to rip. Don't wring the snot out of the engine for the first 1/2 tank or so of gas, and you'll be fine. As CM says, it is recommended to pull the cylinder after break-in and sand any high-spots from the piston, but I don't do this, and have never had any problems.
I probably use excessive caution with my break-ins, but I use forged pistons, which are very seizure-prone during break-in if not done properly, so I err to the side of caution.