Watch the Tuning By Ear! online guitar lesson by Pierre Bensusan from DADGAD Explorer: Guitar Intuite

Tuning is the first step to get you into "listening mode." Look at it this way: before tuning your guitar, it is "you," the first instrument, which needs to be tuned in order to deliver great music from the first note. Your ear is—or should be—the best tuner. It's in many ways more accurate and precise than any electronic tuner, so use it as often as you can, and the more you listen, the more precisely you'll tune and the better the music becomes

After that, you'll find yourself in an exponentially dynamic mode, remembering sounds like pictures that build your audio memory. For further reading, feel free to check out the article I wrote about "The Art/Heart of Tuning" for Acoustic Magazine in the UK.