Don't let the cats see each other right away.
What does this mean? Whenever you bring home a new cat, isolate the animal by locking him up in a different room. If you allow the cats to mingle right away, you take the risk of one of the them getting hurt. You can avoid the claws and teeth by keeping them separated for a few days. Allow the cats to smell each other underneath the door, but do not let them into the same room.
Once they are familiar with each other's scent, place the new kitty in a pet carrier in the same room as your current cat. This will allow them to smell and see one another, while still protecting the new cat from an altercation. Hissing and loud meowing are normal at this point. You may need to repeat this process over a few days.
After the cats seem to be more comfortable in the same room, you may let them roam together. Sure, they may not be instant friends, but they are more likely to tolerate each other.
Give the new cat his own things.
Cats are very possessive of their belongings. Don't expect the new cat to share a food and water bowl with your current kitty. Provide the new feline his own toys, and make sure he has his own bedding. It's even wise to put out an extra litter box for your new friend. Make sure all of these items are new - the scent of a previous animal may keep the new cat from taking to the items.
Make sure your current kitty gets a lot of attention.
Don't forget about your current cat. He was there before the new one, and there is a good chance he is feeling very jealous about sharing his house. Praise your old kitty on a regular basis, and make sure that you give him more attention than before. If you find yourself spending too much time with the new addition, your current cat may grow to resent you. Monitor your cat's day to day actions. If he starts doing something strange, like urinating outside the litter box, he is probably feeling left out.
Most cats will learn how to tolerate another animal over time. With any luck, they will even become friends. Never rush cats into accepting one another. Have a little patience, and your cats will learn to live in harmony.