You have to have an accurate set of monitors to mix on and then you have to learn your system. If you have a decent set already, play tons of commercial released CDs in the style of music (or all styles for that matter) through your set up and learn how they sound. The way the kick and snare are setting in the mix, the bass level, etc. Then reference your mixs time to time to these. When you get to know your monitors this way, it's a lot easier to get your mixes to translate to a wide range of playback systems.