[Old Yeller]

Stolen from the somethingawful pet forums (there are quite a few vets and lab techs that frequent that forum and the regulars usually know what they're talking about):

Introducing a new cat to your other cats

When you bring a new cat into your household, both it and your other cats will probably be upset. You would be too if someone thrust a roommate on you. You can minimize the stress by making the introduction gradually.

Stage 0: Acceptance of terms
There will be lots of hissing. This does not mean you are doomed.

Stage 1: Seclusion
Keep the new cat separate from the other cat(s). Make sure it has all the necessities- food, water, litter, attention. Choose somewhere you can hang out with new cat and get to know it, but not your other cat's favorite hangout, either. The cats will all know the others are there, but this stage allows them to get comfortable with there being other scents around. Try switching out each cat's bedding so they can sniff at it. You might also try giving them their meals on either side of a closed door, so they associate food with the new cat's smell. You can switch the cats out to explore each other's areas and get used to scents & places that way, too.

This stage can last a couple of hours to a week or so. You will have to use your judgment on when to advance. If both cats seem comfortable and curious, rather than totally anxious, continue on.

Stage 2: Supervised introductions
Pretty much what it says. Let the cats meet. There will almost certainly be hissing and maybe even some swatting. It's okay to laugh when their tails puff up. You can let these introductions last as long as you like, depending on how the cats interact. The rule of thumb is not to separate them unless (a) you can't be present to supervise anymore or (b) violence erupts.

Violence is not swatting and hissing and puffing up, or even growling and yowling, necessarily. If blood is drawn or the yowls seem to indicate pain rather than aggression, then it's absolutely time to separate them and return to stage 1 for a while.

This stage might last as little as one or two meetings, or it could go on for weeks. If it drags beyond a week please ask and see if we can make some suggestions to make things go more smoothly.

Stage 3: Kitten piles
As the introductions go better and last longer without incident, you will feel comfortable leaving the cats alone, and from there they'll probably soon reach a level of toleration, if not immediate kitten piles. Kitten piles may never happen, honestly, and that's not your fault. If they do, great!