Use the right tools. A cloth (to rub the gums and teeth clean) is fine for an infant. For toddlers on up, choose a brush with soft bristles and a small head. Use a tiny amount of toothpaste -- a non-fluoridated variety until your child can spit it all out. (Swallowing too much fluoride can cause a harmless but unsightly spotting of teeth.)
Get in there. Until she's 6 or 7, she won't have the fine motor skills to maneuver the brush alone. At first, do it yourself; you can graduate to placing your hand over hers when she's about 3.
Be thorough. Using a circular motion, clean all surfaces of the teeth, and the gum line. If your dentist suggests it, have her chew "disclosing tablets," which dye the plaque and highlight areas you missed.