Comment by CaptainOfCoit

I've done this. The goal is to first demonstrate and reward them, so they associate good feelings with the whole thing. Step two is to kind of put the toy in their mouth, drag them over to where you wanna place it, then make them release it, and if everything worked, you reward them. Do this a couple of times, associate with some command like "clean up" and after a while they'll understand that if they do that they get an reward. After a while the reward can be switched from candies to head-pats, cuddles or whatever, and they should still be able to do it.

You can apply this to pretty much anything and if they're somewhat clever they can do it. Age doesn't seem to matter either, I've done this with 10 year old dogs too.