You can spend all the money you have on great sounding gear, but if your monitors are not set up correctly in a half-decently prepared room you may as well not have bothered. This is because you can only operate your gear effectively based on what you can hear in your particular listening space.

If your speakers are bunched up in a corner of the room, or directly next to a wall - especially if you have speakers that have woofer’s at the back -  you’ll probably find the bass is boosted quite significantly. This is great, until you come to mix your music based on this bass-enhanced sound. When you play your mix back somewhere else, you’ll probably find that there’s no bass at all because you compensated for the ‘colouration’ of your room sound/speaker setup.

Here's how you should set your monitors up:

Make an equilateral triangle between you and the speakers.

Get your speakers as far away from the corners and walls as you can (even a few inches can make a big difference), and try to position them so that there is an exactly equal distance between the left speaker, right speaker, and where-ever your head is when you’re listening/mixing (making an equilateral triangle). You’ll find you can make more accurate decisions about panning and respective levels across the stereo field.

Original source here.