1/2 is now 1/3

Problem : 

We want to simulate a random event that happens 1 out of 3 times (probability = 1/3)
using only a fair coin (probability = 1/2 for heads, 1/2 for tails).


So the job is:

 Flip the coin as many times as you need.
 Make up a rule that says: “Based on the flips, I’ll decide if the outcome is 1 or 0.”
 In the end, it must produce 1 exactly one-third of the time, and 0 exactly two-thirds of the time.
 The process must always eventually stop.

Please describe a clear algorithm, if possible.