They make a special kind of spoon for it that looks like it should do the trick, but I've never used it.
I was taught to do it as a child and have had a lot of practice, so I can't say that my way it the easiest way, but...
Gently crack the egg over a clean bowl. Turn the egg on one end and take off the "top." Most of the white will gush out at this point, so make sure you have it over the bowl and have it tilted slightly to one side so that you don't get too much of it on your hand. Very gently pour the egg yolk back and forth between the two shell halves, being careful not to catch the yolk on the jagged edges and keeping the egg over the bowl. The white will separate out and drip into the bowl. Continue this until all the white is in the bowl. If you should happen to break the yolk, immediately stop the process and pour the yolk into a separate bowl from white.
It's ok for most recipes calling for yolk only to have a little bit of white, so if you got most of the white off before breaking the yolk, you should be fine. It is NOT ok for recipes calling for whites only to have a little yolk in them, so make sure if you break the yolk, you immediately stop trying to separate the white out because the yolk will get mixed in.