I keep a small supply of part-used bags in the garden shed and leave them for 12 months or so. I've got 1/2 a bag of board finish on the van at the moment with a use by date of September 2007 and it's great for patching or other really small sets.
Mix it up and lay on as normal, but it's set solid after about 10-15 minutes. Just what you need or those really small jobs where you want to be in and out as quick as possible.
You can get the same effect by mixing with dirty water, but that can be a bit hit or miss and the set time can be unpredictable.