Puppies are relatively easy to socialize, because they love learning and experiencing the world around them. Dogs, on the other hand, are more complicated. Settled in their ways as early as the age of 1, dogs may resist being around other dogs, and must be conditioned to share the same space. While your dog may never become a social butterfly, you can desensitize her to the presence of other animals and help her adjust to being around unfamiliar dogs.
Keep your dog on a leash. Dogs who were socialized as puppies may be trusted to go off-leash in a controlled setting like the dog park, but other dogs aren't necessarily as receptive to being off-leash around others. Your dog should always be leashed to prevent scuffles.
Maintain slack on the leash as you walk her, even when you approach another dog. Your instinct tells you to tighten the leash so you have more control over your dog, but when you do that, your dog's instinct tells her that trouble is afoot. Keep the leash loose so your dog feels more in control of the situation.
Distract your dog as you pass another one. This may be as simple as saying her name to keep her attention on you or even holding out a stick for her to chase after. This helps her relax and prevents her from becoming too concerned about the other animal.
Reward her with praise and a small treat every time she passes another dog without incident. If she barks or growls at the other dog, get her attention and walk away from the situation -- nobody is perfect, so it's bound to happen sometimes. The key is to reward her for good behavior more fervently than you scold her for bad behavior. Positive association is a more powerful teacher than negative association -- it teaches her that being around other dogs can be a good thing, provided she minds her manners.
Give your dog the opportunity to socialize in a controlled, familiar environment with one dog at a time. For example, invite a friend to bring his dog over to meet yours and take them for a walk together. Even if your dog is excited at first, the extended time together gives her the opportunity to see that this other dog doesn't pose a threat. Introduce them slowly -- don't allow them to sniff each other or get within striking distance unless their bodies are loose and their tails wagging. If one or both stiffen up at the sight of the other, separate them until they calm down.