Is he/she being a little monkey?
Are you pulling your hair out?
Are you lucky to get 3 hours sleep a night??
Don't despair! Help is at hand...
Try "Solve Your Child's Sleep Problems" by Dr. R. Ferber if the problem's to do with sleeping (or, more likely, not sleeping!).
There's also a brilliant book called "Toddler Taming" or "Toddler Training". Once I can remember the author, I'll post it on this page...
My own personal advice (as a parent and a teacher)?
The number 1 rule = CONSISTENCY. If you say "no", mean it. If you're not going to follow it up, don't say it! It often pays to just bite your tongue for a second and think "Do I really want to say no?". That way, NO means NO, and it will retain its power.