Most common one I've seen is the little briggs sachets that do 5l of fuel for about $4. They are becoming available everywhere - mower shops, bunnings, etc. I just get 5l of fuel, use for a month or 2 and then put the remainder in the car and fill up again.
I only keep 2.5l of premix at any one time (1l 25:1, 1l 50:1 and 500ml of 40:1) - but I can understand why some people prefer to mix 5l at a time and use it over a year.
