One great way to thin your kid's toys before Santa pays a visit is to perform an exercise I like to call "shopping for favorites." If your child is over age five, take all the toys out of their bedroom and line the hallway with them. Then hand your child a bag and have them "shop" for the toys they love the most.
The reason this technique is so effective is because removing the toys from their context (i.e. the shelves in your child's bedroom) puts the toys in a new light and they will either be rediscovered, or pale in comparison their toy neighbors.
If your child tires of picking favorites, shift their focus to the other end of the spectrum and have them select toys that don't make them happy anymore. When they again tire, back to favorites. Your donate bags will be full in no time, your child will be reacquainted with their things, and they will have gained space for new interests!