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.