A: The key is to learn exactly what your monitors and room sound like, so that you can mix accordingly. The best method for this is to listen, listen, listen, and then listen some more. Listen to both commercial mixes (in a wide variety of musical and production styles) and also listen to your own productions. You must get to know the sound of your system intimately. Take those same commercial mixes and play them on other systems. Make notes of the sound and balances and compare with the differences you heard with your own mixes. Then go back and listen, listen, listen some more!
In addition, verify that the acoustic treatment in your room is producing the results you need. If your mixes are coming out bass deficient, then it is likely that your room/system is low-frequency heavy, causing you to not put enough low end in your mixes.