One of the hardest things to do is to get kids to give up their old toys. Not because they actually use them, but because, often times, they have great memories associated with them and they are sure they will want to play with them again.
An easy way to get them to clean out their closets and toy chests is to have a goodbye party for them. Ask your kids to round up all of their old toys that they have not played with in a while and have a full day of play with those special toys. Give them carte blanc play with them in the living room or outside for a few hours, maybe even have a goodbye cake as a treat and then, after the goodbye play is all done, have buckets of soap and water for them to clean the toys to go to their new owner.
Having volunteered in a charity thrift store, I have seen bags of broken and dirty toys come in and making them look fresh was very time consumming and ends up taking away from the time we could have spent getting the toys to the proper new owners. By donating clean (and minorly repaired) toys that have been gently cleaned by loving pre-owners, the recipients will get a toy that is already in great condition. This also helps the kids see how valuable the gift they are giving away is an