The "drop it" command is probably one of the most important things you can teach your dog. Dogs like to pick up all sorts of things; some eat weird stuff and other love to chew up anything and everything. It is imperative that your dog have a very solid "drop." Unfortunately many dogs resort to the swallow fast option when asked to hand over an object that is in their mouth. These are the one's that need training the most; as there are literally hundreds of dogs that need surgery because of what they swallow down in a panic.
The best as always is to start when you first get your dog; but you can start this at anytime. One of the important things to remember is to never, ever chase your dog down for an item. This is a game and one that most dogs love; dogs will go and find something they know they are not allowed to have so that you will chase them. For the swallowers; it is the chase that makes them swallow. They would rather swallow it down than hand it over.
What you have to teach your dog is that if they hand over an object; you will reward them. Most rewards come in the form of a treat but if your dog is not motivated by food you can use anything; a ball, a pull toy or a squeaky toy etc. You must train this when your dog has things that you can return to them; you need to teach them that you don't want to take their things from them. The object of the game is to teach them that dropping things is a good thing; drop the item, get a treat and get your thing back.
Typically in the beginning of training you need to bribe with a food item while associating the word "drop" with it. When you do enough training with items you can return to them; the couple of times that you cannot return an item do not have as much lasting impact. This should be done several times a day and even once they have a great drop, keep it up every so often to keep it fresh.
Drop can save your dogs life; or at least a trip to the vet, or a bout of vomiting and diarrhea. If they drop something amazing; like a bone or coyote poop make sure to go over the top with praise and a treat if you have it. My guys definitely like coyote poop and if they eat it, sure enough we have a day of throwing up dogs. So if I can catch them before it is swallowed, I'm impressed and very happy.
Make sure that "drop" is one of the things you teach your dog.