It will never be as clean as it when it was brand new, but you can do things to make it at least look cleaner than it is now. What I do is i fill it with rubbing alcohol and set fire to it, wait until it starts to boil (should take about 5 seconds or so) then put it out with water. After that I put half the rubbing alcohol I had in it before then shake it, do that a couple times and it will look much cleaner.