My Green Australia Quick Tip:

When cooking with egg yolks and you have leftover whites, pop the whites into a ziplock bag LABELLED CLEARLY and put it in the freezer.  You can add egg whites to that bag till you fill it, just change the number on the label as you add.  Frozen whites are just as good as fresh ones but make sure no moisture at all leaks in as this prevents them from whipping. Before long, you have enough for a mega pav!

Any other great tips for egg whites, let us know!