As your children continue to get more toys with each passing birthday, Christmas and other special occasions, space becomes a premium, and clutter can begin to take over. So how do you keep it in check and still enjoy giving them new things? You have to balance "out with the old and in with the new". If there are toys that are broken, or games and puzzles with missing pieces, they can probably be thrown away.
Why not set aside different times of year that you go through all of the toys and decide what needs to be given away? This can be a month or so before birthdays and Christmas. However, if you wait until after they get new things, they might be more willing to part with the older toys. It is important to talk to the children about it beforehand, if they are old enough to understand. Let them help decide which toys they might want to give away.
As the children go through their toys, let the younger siblings see if there is anything they'd like to keep from their older brothers or sisters, but if they do they have to give up something from their toys. Try to keep it even with how much the kids are getting rid of, as you don't want them to feel like they are sacrificing more than the others.
If the weather is nice, you could even encourage your children to participate in a family yard sale. They get to keep the money for any of their things they sell. This takes a bit more thought and organization. First, they have to make sure that what they are selling is in good enough shape to sell, and that all the pieces are there.
One important thing to keep in mind, especially with the younger children, is to give them time to adjust to the idea of giving things away. Young children can often be generous with their things, but then later realize they gave away a "favorite" toy. Let them practice for a few days, or even weeks. Put the things away in a box where they can't get them. If they realize they don't miss the toys, then they should be ok with going ahead and getting rid of them.
It is also good to encourage sharing their blessings by donating some nicer toys and games they've outgrown. There are many charities that take gently used books, toys and games, including hospitals and shelters. This is a great way of not only reducing the clutter in your home, but also helping someone less fortunate.