The first thing you need to look at is your listening environment. Having good speakers is only one small part of knowing you are hearing everything in your music, especially with any sort of accuracy. Far more important is the way that the sound from your speakers interacts with the room you're in. The best speakers in the world can sound like dog poo if your room is negatively influencing what you hear. [...]
The best way to get around this limitation then, is to make sure that you listen to your song on as many speakers and playback locations as possible. For the longest time my car was my second test location, as I by far listen to more music there than anywhere else. But don't stop there, try your song on your iPod, your home stereo, your kitchen radio, your friends' stereos (they hate this by the 5th or 6th month BTW), etc. Learn how to translate what you hear in your home studio, into what it sounds like elsewhere. Pay attention to things like the bass. If it sounds good in your studio, but you keep noticing it's too bassy elsewhere, then you know you need to compensate for that at home. So you mix and master with the bass sounding weak at home, so that when you test it elsewhere, it sounds good.
Do you like it ? Well, read the whole guide :)