You can use clove oil, it works as an anesthetic. You can get it online or at health food places like GNC. There is a method where you use clove oil to put them to sleep and then vodka to help them pass on. You can also use clove oil by itself. Once their asleep, you just add a higher dose and that will do. That's how I euthanized Whiskey recently. Some people use quick blunt trauma.
I've used blunt force trauma, giving both a sick goldfish, and a sick betta a quick blow to the head with a shovel. I simply removed the fish from the tank and wrapped it up in a damp cloth to lessen the stress of the situation. It was all over in a few seconds and I'm sure that it was worse for me than it was for the fish.
The only other methods I would recommend would be the use of clove oil, or something immediate like decapitation. There are still mixed opinions on whether freezing a fish is humane or not, and personally, I would like to know that death is instantaneous rather than prolonged.