we've tried to persuade Rufus to drop his ball for ages, then the other night he just did it- he was walking up the living room, drop it, take it, drop it, take it... etc - it was hilarious (and very clever too!)
Trouble is now, when you ask him to bring you his ball he picks it up and puts it down before you can ask him to drop it
sadly he learnt this skill after the pigeon and dead fish incident

- not sure how it will work with high value stuff.
I think he got the hang of it because we started off having to take the ball from his mouth, (whilst saying 'drop it') persuading him by showing him a treat - then it progressed so that if you got hold of the ball in his mouth he would release it to your hand - and now he can let go all by himself!