I hold on to any old duff fuel and use it for cleaning off engine parts, especially carbs etc. as unlike cellouse thinner, old fuel won't affect any of the rubber seals on the carb and needle assemblies, obviously the componants will need a light oil afterwards to stop any corrosion before reassembly.
Whats left over afterwards I use to start the next garden bonfire, just be sure to stand well back when you light it, as it will go up with a "whumf" when it catches, and the flame is almost invisible!