How to Properly Position Monitors (Speakers)
From WikiRecording
For the true beginner out there, "monitors" are the audio term for speakers you use to monitor your mix.
Monitor Placement
Before you begin to mix it is crucial that you have placed your monitors and are sitting in the proper place to hear accurately. Monitors should be equidistant to each other and you. Basically think of yourself, and the monitors are each 3 corners of an equilateral triangle. Sometimes it is easiest to grab a tape measure and check, alot of engineers will then mark off the sweet spot on the top of the desk where the actual center is. You should also make sure that you are roughly level (at the same height) with your monitors. Sound works in 3 dimensions not just left and right. It is usually good to point the monitors somewhat inward toward you, but the amount depends on the stereo separation you desire. If you are having problems with your monitors buzzing it can be benefitial to separate them from the surface they are touching. You can purchase foam pads specifically for the job or if you are wishing to save some money go to a carpet store and pickup some scraps of the thin 1mm thick mesh foam used to make carpets not slide across the floor.
Monitor Level:
Technically if you asked the AES (Audio Engineering Society) they would tell you to monitor at 80db SPL(sound pressure level). However not everyone is an audio geek sporting a device that can measure SPL's. The real importance in the number is that anything above 85db SPL can damage your ears over prolonged use. Another reason not to monitor at really loud levels is that you hear differently at different levels. The Equal Loudness Curves that are linked to at the bottom of the page show the various differences in EQ your ear applies at different loudness levels. It is a good idea when you start a session to mark off a few different levels on your main CRM or master fader to use as reference levels when you mix, this prevents you from "creep" as your ears get tired you won't be tempted to keep turning up the volume.
Keep in mind that alot of times a change in volume will completely change your perception of what needs to be changed in the mix, resulting in an ever increasing movement of the faders upward until you have no headroom and no where for the faders to go.
Links
Graph of the Equal Loudness Curve