I hate teaching the holding part of the retrieve. I save it for last as I think both the dog and I have more fun when we're moving around. (Click here for my retrieve training). However getting a solid hold on an object without mouthing is important in the obedience ring and a necessity for the service dogs. No one wants a punctured juice box brought to them!
Luckily the majority of dogs won't mouth if they are moving fast enough so for most service dogs you can get by with just focusing on a solid retrieve to hand or lap as long as the dog is really working at pushing the item into the persons hand. Without formal hold training the dogs can also usually heel with an item in their mouth and also do a quick rise up to a desk and drop/bring an item without mouthing as neither task requires the dog to be standing still. However I still would love for all the dogs to have the ability to sit or stand and hold an object without crunching it to death or rolling it around in their mouth so I try and teach a nice hold if I have time. And with my dogs it is a very fun trick and great for pictures!
I find it very time consuming and have yet to find one easy force free method that will work for the majority of dogs. I use a lot of experimentation and am continuously looking for new ways to explain to the dogs what I want. Here is the list of ideas I have successfully tried, please comment on other methods that work for you! Also note that some of these methods won't work unless your dog already has a retrieve.
*db= dumbbell, but start out using whatever item the dog enjoys retrieving.
*with any method the click ends the behavior so I could care less if it falls to the ground when I click.
*Remember to click when your dog still has the db in its mouth. If anything try and click early as being late with your timing will just reward the dog for dropping it.
Edit: Apparently a variation of this method has the name reverse luring. I've since had good success with the open/closed fist in teaching the hold.