Whenever I mention the term "rude" in terms of dogs (a term I first heard from my friend Janey at Rogues and Rascals--Thanks, Janey), I can see people visibly recoil. "Surely my dog isn't rude. He just wants to say 'hi,' but other dogs are always attacking him." Tell you what: If you find yourself saying that, chances are, you've got a rude dog.
The problem is, rude dogs look pretty friendly to the human eye. After all, they run up to other dogs and greet them with a friendly nose sniff, tail wagging. That's not rude, is it?
In dog language, a dog that approaches another dog directly while openingly looking into his face is challenging him. Chances are, that dog being greeted will snap or lunge to indicate its objection to this overly familiar and challenging greeting. To start getting this under control, ask your dog to sit and face you when another dog is approaching her--and praise her lavishly when she does. If you do let her "greet" another dog, make sure their greeting is respectful. Let the dogs sniff one another from behind, and never, ever let her approach strange dogs by going nose-to-nose. When a dog graduates to a nose-to-nose greeting after multiple meetings, watch for signs that either dog is becoming agitated, and don't let them greet one another for more than a couple of seconds.
To build safe greeting behaviors, help your dog focus on you, and create situations to help her succeed.