Rolling at a moderate to slow speed, pop an ollie, but as you kick your front foot for the "flip," swing your back foot underneath and behind you 360-shove-it-style. This will rotate the board around as it spins. The whole thing should take about the same amount of time a kickflip does, so you won't have to "hang" in the air any longer. When the nose comes back around and the griptape side shows upward, stick your feet back on and land it.
The 360 flip is what you might call a "compound trick" because it actually combines two tricks, the kickflip and the 360 shove-it. To do a 360 flip, you should position your front foot like you would for a kickflip, with your toe slid back toward the heel-edge of the board. Your rear foot should be positioned with your toe on the opposite corner of the board, namely the toe-edge of the tail.
Click here for Quicktime (640k) or here for an MPEG (280k) video clip of the 360 flip.