Like rangeroger, I use an A440 tuning fork. I've been using that method for so long now that I've got a pretty good sense of the pitch of an A harmonic (which I strike on the D string) so that I can get away without using the fork pretty well now.

I don't find electronic tuners very useful on acoustic guitars. They're OK on the top four strings but below that they're too unreliable. I think you get too many harmonics from the higher strings and that confuses them. Just a theory.

By the way, I hear that there is a fish that lives in the South Pacific whose scales vibrate at a constant 440 hz when it is caught and taken out of the water. Apparently it's called the Tuner Fish....