You can buy different types of tools to remove the lockring from a fixed gear track bike. Some are the spanner type and some are pliers. I like the pliers best but I don't want to shell out $30-$50 for a set so I decided to make a set.
Step 1: Gather your weapons!
First thing is to buy some pliers. I bought a cheapo set from Menards for $5 and they worked great. Just make sure you buy a big enough set that they'll open wider than the locking and still have the jaws parallel to one another.
You'll also need something to file the metal, I liked to use a hand file but you could also use a dremel tool with a stone wheel. The cresent wrench in the picture is to take the pliers apart, you probably don't have to take them apart but I thought it made it easier.
Step 2: Dremel or file the teeth
Get to work cutting down the teeth, leaving the front bit of the pliers to fit into the notches of the lockring. Take your time and try to get them to fit as tight as possible and get the teeth to fit into the notch as far as possible so that they won't slip off.
Step 3: DONE!!
When you get done they should look something like this. When you go to use them just fit them in the notches, give them a good tight squeeze and crank the lockring off. Remember that the lockring will be reverse threaded, don't rip the threads off of your hub.