Frisbee Flipping

So, a question that I found on the intertubes some time ago was re-inspired by talking with Andres. In a game of ultimate frisbee, the teams must flip a coin to decide who goes first, however, a fair coin is not usually available. What is available are frisbees, so they try to use them as coins. Its not hard to guess that a frisbee does not make a great coin, as it is biased to land on one side over another. This gives us the problem to solve:

You are given a biased coin that lands on heads with probability k and tails with probability 1-k (k ∈ (0,1)). Without knowing the value of k, how can you use this coin to simulate a fair coin?

I'll post the solution next time.