Near Field Monitors:
General consensus is that they should be at eye (or ear) level, about one metre apart and about one metre from your ears when you're sitting at your console. That way ensures that what you're hearing is affected less by the room acoustics.
Other monitors:
Depends on shape of room, surfaces, furniture, bodies - not enough space here to elaborate. Check out "The Handbook for Sound Engineers: The New Audio Cyclopedia by Glen M. Ballou (Editor)" (ISBN: 0240803310); or, if your budget doen't go to $100+ try "Hot Tips for the Home Recording Studio by Hank Linderman" (ISBN: 0898796512) which is about $17 from amazon.com.
Hope this helps -
- Wil