As important as selecting the correct set of monitors is, locating them properly in your studio is equally vital - there are many variables to consider regarding acoustics and the position of the monitors relative to the listener. In particular, when using 'midfield' monitors in larger studios or control rooms, special consideration should be given to acoustic treatment in creating the ideal mixing environment.
Reflections from walls and other surfaces in the room can be misleading and distracting when mixing, although this is generally less critical when using 'nearfield' monitors as the listener is positioned much closer to the speakers, reducing the audible effect of the reflections. However, both near-field and mid-field monitoring systems can suffer from reflections originating from the rear of the monitor, which are especially prominent with rear-ported reflex systems. These reflections can lead to exaggerations or cancellations at specific frequencies or frequency ranges, and can have a strong and undesirable influence on the overall sound. Experimenting with monitor position relative to the rear and side walls can minimise this problem, and the addition of acoustic dampening material around the studio will also help -bass-traps in particular. Even moving the monitors by only a few inches can have profound effects on the perceived frequency response.
The process of monitor placement itself can be rather hit and miss and time-consuming, but you can assist the process by playing simple test tones through your speakers from a sampler or sequencer. Set up a range of low frequency tones of equal amplitude over several octaves - on playback some tones will sound different in amplitude to the others, and by moving the monitors around you should be able to make most of them sound equal to the others. Some higher-end monitoring systems incorporate a reference microphone and test program for automatic DSP adjustment to compensate for differences in room acoustics and placement.
Correct height positioning relative to the listener and also sturdy placement are both often overlooked, but are very important if you want to get the most out of your monitoring system. Ideally, tweeters should be at ear level and slightly angled in so that the monitors are directed at the listener. The physical mounting or structure supporting the speakers is also worth spending some time experimenting with - products like acoustic deadening material for hollow speaker stands, and spikes or cones for under the speakers are all available, and can help you to achieve optimum monitoring conditions, particularly in terms of low-end performance and stereo-imaging.