I use some airline and a small fitting with a screw-valve I got from the LFS. Once the syphon is started, closing the valve doesn't kill it so it's very easy.
I'll drip some in, close the valve, drip some more in 10-15 minutes later, close the valve, drip some in . . .
Every once in a while I'll also pull some water out of the acclimation container so that the proportion of my water slowly gets greater and greater.
Also, my acclimation bucket is two Tuperware containers, one inside the other. I drilled a bunch of holes in the bottom of the inner container, so when it comes time to put the fish in the tank, I can merely lift the inner container out, the acclimation water drains out, and I can either pour the fish in the tank or place the container in and let the little buggers swim out. This reduces to an absolute minimum any potential of the store/shipping water getting into the tank.