If you are using a slip chain and your dog is choking take it off now and never use it again. They are really dangerous for dogs and can cause all kinds of problems.
ready this
http://www.gopitbull.com/obedience-training/38918-prong-vs-slip-dangers-slip-chains.html
I am a trainer and I use prongs all the time and they can be a great tool but for the right dogs. Lets look at collar options for dogs
flat buckle collar- great for a dogs ID tags and fine for sensitive dogs but hard pullers will never walk well till after they are trained.
Slip chain, 95% of people use them wrong and the the does end up choking on them, they are also dangerous and can cause fractures of the neck and spine, see above link.
Harnesses do not teach the dogs not to pull and you have no control over the dog.
easy walker harnesses or no pull harness can jar a dogs neck and back out of alignment and cause damage to the spine, no recommend.
Head Halties- same thing they can cause sever neck and nerve damage by pulling the neck out of alignment, also spine damage.
Martingales- There are my next go to collar they are pretty safe and work by putting pressure around the neck evenly just like a prong collar but without the prongs.
Prong collar- Best option for a hard puller and stops pulling if used right with in a few corrections. Some times prongs are too corrective for some dogs and in that case you look at flat buckle collars or martingale's.
When using any collar on a dog if you want to stop pulling there are several ways to do so. The easiest was for hard pullers is to jerk and release with the leash to correct the dog for getting too far ahead. It is important to jerk or pop with the leash not just pull back. If you still have problems then enroll into an obedience class to get some more help.