I would practice the "give" command....I should really practice it more but Maximus has the general idea..although it takes him a sec or so longer if he really likes what he has
....what you do is drop something appetizing and hold out something even more appetizing while saying give..when they let go of the original thing give them the treat...repeat this and give the treat every other time..repetiton is key and then you can practice in different settings. I for one never realized how much people litter until I had Maximus and he is constantly picking up dirty crap off the street so we go through this about every morning!!