Training your ear is like training your fingers on the guitar, it takes repetition, and it takes accuracy. Tuning your guitar every time you play is the simplest and most natural way to teach your ear what sounds good and not.
There are some excellent tools you can use in addition to just tuning your guitar regularly. My favorite is the Advanced Ear Training Software for musicians from Jana Software. For more about Jana Software make sure to check out the information on our main website.
Another strategy it to listen to your favorite recorded music. People tend to hear the music, what I am talking about is listening to the music. Focus on the sounds you hear and how they change. How the tones go up and down. Don’t worry about what key the song is in, or what notes is being played, just focus on the tones.