If you are referring to a prong collar, then it's very important that the collar is fitted correctly and that you are using it correctly, like Chance's Mom said.
I've found that many store employees who recommend (and fit) those collars for customers really don't know how to fit them correctly, and many of the dogs I see with prong collars have the collars hanging loosely around the bottom of the neck where they are completely useless.
The link Chance's Mom gave shows you how to put on and fit a prong collar correctly. A lot of people don't have them all the way up behind the ears, but you do want it to sit around the upper part of the dog's neck, not at the bottom.
When you are using a prong collar, it's important that you're not simply letting the dog self-correct (hit the end of the leash to get a correction) but that you are using the collar as a training aid rather than a training substitute.
That said, your problem seems to happen when you come toward another dog. In that case, the prong collar may not be a good solution because it can actually ramp your dog up and make the behavior worse, rather than better. What you want to do is a way of redirecting the dog and/or catching the dog before he reacts to the other dog on walks.
Is your dog at all motivated by food (treats) or toys? If so, you can use that motivation to redirect his attention by having him do a command when you spot another dog (but before your dog spots the other dog). Have your dog do a command such as "focus" (look at you) or "sit" so you have his attention and he is focusing on you and waiting for a reward, rather than focusing on the other dog. You can also try walking in a different direction or doing a turn.
Quote:Go to a pet store with your dog and find one of the clickers.. See if your dog responds well to it.. That is something else you could try. Just redirecting the attention...
The clicker is a great training tool when it is used correctly. If you expect the dog to respond to the sound first time he hears it, or using the clicker to redirect the dog's attention, you are not using it correctly.
A clicker is only a marker that marks when the correct behavior occurs and lets the dog know that yes, this is what I want you to do, and a treat is going to follow. Of course, the dog doesn't know that click equals treat until you have "charged" the clicker, meaning you've conditioned the dog to understand that a click will be followed by a treat.