I try to mix only enough fuel (alcohol) to ensure that I won't run out at the track and used to dump what was leftover but after talking to Klotz and Benol I quit wasting the fuel. Both told me that as long as the fuel was in an airtight, sealed (preferably steel) container off the floor that all it needed was a good shake and it was good to go. I've since used fuel that was held over from one season to the next. As old as 8 months with no problem. I strictly run asphalt and my ET slips have never indicated the fuel had gone stale.
*RadarRacing is correct that you don't want to leave alcohol in your vented fuel tank*
On a cool Ohio night when I purge at the end of the day I can watch the fuel in my drain pan turn white within minutes from moisure in the air.