"Leave It" and "Out" (or "Give) are two very important commands, IMO. But they are also among the more difficult to train. We used the "trade" method outlined by LuckyRescue above. Hey, we don't have nearly as many distractions as Moontamara does, and it's still a bit of a struggle for us on some days!
Another thing we learned that I've ingrained in Phoebe's training is "Wait". This is used for a variety of purposes (walking through doorways, getting in and out of the car, etc...), but I also use it when feeding. Phoebe is not allowed to eat until I give her the release command. Today, she got leftover beef stew for lunch. I gave the "wait" command, and poured the stew into her bowl. WOW! Was she ever staring at me intensely! But she was a very good girl and waited until I gave her the go-ahead to eat.
This, in my mind, reinforces to her that I am Top Dog and the giver of food and praise and all things good, and so she would do well to listen to everything I say.