Dogs are like kids -- you need to teach them how to behave and act in society. Doggie society, in this case. When trying to work on Fido's social skills, keep in mind that puppies tend to be a lot friendlier and more open to meeting other dogs. Once dogs get to be a year old or more, other dogs are no longer potential playmates -- they're just strangers he's not sure he can trust. So thread lightly and take it easy to avoid fights.
Spay or neuter your doggie, unless you don't mind all the humping to come. Male dogs will not only try to hump females, but also other males in an effort to establish dominance. That's a fight waiting to happen. If you plan on taking your doggie to the dog park on a regular basis, this is a must.
Leave favorite toys and treats at home. Not all dogs are good at sharing -- especially with strange dogs. You don't want to be playing referee or trying to convince Fido it's OK to let other dogs touch his stuff. If things go well at the dog park, they can always play catch with twigs or find other ways to entertain themselves. No need to fight over a rubber ball.
Keep Fido on a leash when you first meet other dogs. This will allow you to control his behavior until you're sure he's just trying to play -- and not eat the other dog alive. If you have a strong dog or one that starts to pull like crazy every time something interesting happens, you need a harness. This will prevent you from strangling your doggie as he starts pulling to say hi to the other furry ones around.
Set up play dates for Fido. These are great for helping Fido practice "acceptable behavior" in a controlled setting. You don't want to unleash doggie into a dog run when he doesn't know what "proper behavior" means -- that's just a recipe for disaster. Play dates offer a more controlled environment, where you can easily say "no" or "stop it" when things get out of line.