I think this is the first year we've had to refill feeders before the food had time to spoil! So I was looking up the recipe for the umpteenth time just now and figured it would be a good idea to just post it here.
Use one part ordinary white cane sugar dissolved in four parts water.
It's not necessary to boil the water. The microorganisms that cause fermentation don't come from the water; they are transported to the feeder on hummingbird bills.
Store unused syrup in the refrigerator for up to two weeks.
This mixture approximates the average sucrose content (about 21%) of the flowers favored by North American hummingbirds, without being so sweet it attracts too many insects.
The recipe was taken from this site, which has a lot of great info on hummers, plus a really cool video of hummers at the feeders.