Generally speaking, the top is first choice, anywhere from a couple of fingers width in from the rim, a couple of finger widths high on about a 30 degree angle. Or my fav, 6" out from the edge, around 3" from a line even with the top of the drum aiming at the rim/top. Aiming at the side of the shell an inch or two away can be cool. Experiment.
if you mic the batter and then you mic from underneath, make sure you reverse the phase on it. Snares can sound really nice with two mics. Use a 4:1 compression ratio too and play with the attack and release settings to get the sound you're looking for.
Once you have it "dialed in" and your level is set, you can use EQ to make it "jump" out of the mix and have it where you want it with all your other instruments.. but everything trackrat said about micing from the top is right on.
Sometimes you can place say a shure 57 closer to the 8'oclock posistion to pick up more ghost notes, though if you miced from the bottom you'd get so many ghost notes you could raise the dead.
Question here, I have my mounted toms right in front of my snare on a stand so I don't really have mic access to it, suggestions? P.S. I REALLY don't want to have to move my toms to record.