I've done music all my life so I have a very good ear for it.
I've always mixed by ear however, I would like to know where to get started to truly become efficient.
I use reason 5.
1) Have a treated room, or at least a room where the acoustics are pretty balanced.
2) Spend quality money on a set of studio monitors. Some like the high-end Mackies, whatever. If you ain't spending $500 or over per monitor, you're likely getting substandard shit.
3) DO NOT MIX USING HEADPHONES.
4) Mixing is all based on the ear. You hav to practice, practice, practice. There are tutorials and boards, but usually it will take you months to even get a grasp on how to get a good mix.
5) There are two camps of mixing, the British and the American camps. The Brits use EQ automation to mix and duck frequencies moreso than the Americans. Americans usually just pull volume up or down. Try to learn both.
Good luck. Mixing is a profession in and of itself.