I have been using a “shaker siphon” for 10+ years to fill up cars, boats, generators, lawn mowers, etc. The problem with fuel cans is that when they are full they become unwieldy to pour from a spout. The best method I have found is to siphon fuel from the can rather than pour it.

To use a “shaker siphon” put the copper end into the fuel can and the tube into the tank you want to fill. Then lift the copper end up and down (shaking it) in the fuel can. The entire process takes seconds. The system works best if the fuel can is higher than the tank it is filling. Afterwards a siphon is created which transfers about 3 gallon or so per minute from the fuel can into the tank. You will hear a bit of jingling from the copper end which lets you know it is working.

I can typically get almost all of the fuel out a can with a few ounces leftover. Those last few ounces are easy to pour out. Lifting the copper end out of the fuel can stops the transfer immediately. When you are done, it is best to lift the copper end above your head while the other end is still in the tank to let any fuel in the line drain into the tank.

I use the Hopkins FloTool Shaker Siphon. There are several variations of this design on Amazon and from what I can tell one is good as the next.

via http://kk.org/cooltools/flotool-shaker-siphon/