1. Learn to sing what you play.
You may have heard the old saying "if you can sing it, you can play it." This is absolutely true. Being able to sing what you play reinforces the connection between your brain and your fingers and leads to more accurate guitar playing and faster interpretation of musical passages.
2. Play by ear.
This is an obvious one, but a very important one. Be sure to spend some time each day learning riffs, solos, or songs by ear. Sometimes I like to sit with my guitar while watching TV and try to learn the music I hear during the commercials. It can get pretty challenging!
3. Learn the vocal melodies of your favorite songs.
This is a great ear training exercise and not only develops your musical ear, but can lead to interesting licks and phrasing that you can add to your guitar playing repertoire.
4. Quickfire note identification.
Start by singing a random note. Then try to find that note as quickly as possible on your guitar in as many different octaves as your instrument allows. The idea is to be able to instantly play a pitch as soon as it enters your mind.
5. Learn to identify musical intervals by sound.
This one is a bit more academic, but comes in handy as you increase your knowledge of music theory. For example, being able to hear the difference between a major and a minor 3rd will give you the ability to distinguish between major and minor chords.
These are just a few of the many ways you can develop your musical ear. Remember, a good ear is your best asset as a guitar player. Have fun with it and try to come up with other ways you can work on your ear training.