The same basic drum tuning principles apply to snare drums as apply to toms.

The big difference with the snare drum is, well, the snares and a thing called a snare bed and the very thin snare side head. Because of this and because of the expected sound of a snare, the tuning is different in the following ways.

A- The snare heads are usually tuned much tighter than the toms. The reason is two fold. First, the sound usually associated with a snare drum is a tight, bright one. Second, for the bottom head to be responsive for the snares, it needs to be fairly tight.  I've heard from a number of experienced tuners that good starting point, for those of you with a pitch pipe, is to tune the top head to a middle G. 

B- The bottom head should not be at the same relative tension as the batter head. In order to reduce resonance, and to help reduce sympathetic buzz of the snares, you want a good separation between the top and bottom heads. Sympathetic buzz is when the snares 'sing' along with the bass guitar or your monitor speaker.

C- Snare Bed: The bottom bearing edge of a snare drum, has two dips or cut-outs in it, where the snares come across the head. This is to allow the snare wires to push up into the snare (bottom) head. Because of this, it is difficult to get even tension across this head. Don't worry about it.   Without even tension in that head, it will be less prone to sympathetic buzz, because it won't have any specific fundamental note. Tighten the snare head above or below the the batter head, then tighten the lugs directly adjacent to the snares, until the wrinkles come out. It will take some experimenting to get the relative tension that works best for your drum.

For most styles, you want the snare head very tight. Not only does it give you that nice crack that most of us love, but you get the added benefit of greater response from the head. Loose snare heads have a place, too. You get that soft, almost lagatto, snare sound that sort of goes, "poof." The theme of all this is suit yourself