Watch the Week 5: Tune Your Ear online guitar lesson by Roberto Dalla Vecchia from Flat Out Flatpicking
First get an electronic tuner, then learn how to tune without it! Don't use a tuner to avoid using your ears. They are a great time saving device and I couldn't imagine playing a gig or recording without one, but they can make one lazy. Don't assume that having a tuner means you don't have to learn to tune by ear. The electronic tuner is the perfect tool to train your ear! If you do this simple exercise for a few months you'll be surprised, it will lead to stunning improvements in your ear.
Just follow these steps:
1. Tune a string of your choice with the tuner.
2. Tune the other five strings by ear. No hurry, try your best.
3. Check how you did with the tuner, take a mental note of your mistakes.
4. The next day, repeat steps 1, 2, and 3 and try again.