We know that once our kids do something that they find rewarding it is very difficult to get them to stop. Hoarding is one of those behaviors. The best way to handle hoarding behaviors is to stop them before they happen. If you see your child starting to collect certain items try to encourage them to diversify their interests. Our kids are more likely to find ways of releasing anxiety if they have a broad array of activities and less likely to seek stress reduction in the form of hoarding. For many children, hoarding equals safety. The more they have of their desired items, the more secure they feel. You will know if this behavior has become problematic if you see your child compulsively collecting items, stealing to obtain certain items or refusing to part with them.
Like other compulsive behaviors it is often unhelpful to nag or constantly ask for the behavior to stop, but there are some things that you can do.
Set a standard for cleanliness. Take a picture of their room in a tolerable condition, if the collection goes beyond what you consider acceptable then items need to be discarded or donated. Doing this allow for some collecting but the expectations are clear and predictable as to what you will allow and not allow.
Put it to good use. I have known individuals to make jobs out of their hoarding tendency. One woman collected coupons and made a job out of clipping and sorting for her neighbors. Another woman had a small business recycling cans and bottles.
Appeal to their sensitive side. Children will be more willing to part with their hoard if they know that it will be put to good use by people in need. Local libraries will usually accept donations of magazines, local shelters will take clothing, recycling centers will accept newspapers. Doing a good deed for the community or environment could be just the motivation needed to clear out collected items.
Hoarding takes on many forms but is almost always connected to a lack of control over ones life and a need to feel more secure. Look at what areas of your child's life could be lacking in these areas and you will be well on your way to getting a handle on this problematic behavior.