From David Stewart, Sweetwater's Service and Technical Support Manager -

Common question: "How close should I position my "near field" monitors?"

Our general rule of thumb: If they are smaller speakers, put them out about arm's length away from you, with about that same distance between them (about one meter). They should be pointed in towards your head. Envision an equilateral triangle with one of the points being the tip of your nose. The monitors should also point towards the tip of your nose so the on-axis sound kind of crosses right in front of your face. Your results may vary and personal preference should always take precedent over these rules, but this is a good starting point; from here adjust to your taste.

This method generally works well with small two way (or dual concentric) near fields. If you have larger monitors or three way monitors the space between the drivers on the cabinet may dictate that they be placed a little further away. If you sit too close to monitors with multiple drivers or monitors that have a large distance between woofer and tweeter you will experience phase distortion anomalies.