How to Discourage Thumb Sucking – eHow.com
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“Children who suck their thumbs after around age 6 may become the target of teasing and criticism; they may also end up with dental and speech problems.

Steps

Wait for the problem to go away. Most children, but not all, outgrow the habit by the time their permanent teeth come in (about age 6). After this age, thumb sucking can cause dental and speech problems, but before then, many children still have a fairly strong impulse and need to suck.

Start to work on kicking the habit a little while before you expect your child’s permanent teeth to come in. That way you can deal with it before it becomes a real problem.

Try to keep your child talking, which will make it hard for her to keep her thumb in her mouth.

Offer her activities to keep her hands busy: crayons or pencils and paper, squishy balls or toys to fiddle with, or models and puzzles to work on.

Consider working out a reward or praise system, using stickers and a chart or calendar.”