Water-man, any advice on how to do this?
Pine cones with real wings tied on them work well to teach a soft mouth.
I started my lab on top of a carpeted 55 gal barrel. I gave him the command "whoa" as I walked around him and slowly increased my distance from him. The pup will be unstable on the barrel and it will make him want to sit and not come to you. I'm with gamestalker on the importance of stopping a retriever in his tracks. It has saved my dog more than once.
Whatever commands you choose to use, keep them constant and only use them for training/field work. Don't mix training with family dog business.
I also work with an orange whistle. When my dog sees it, he knows we're going to train or hunt.
The commands I found to be important are, Whoa, Come , Sit , Fetch it up, Hunt it up, and heel while we're moving from one local to another.
I use the fetch it up after the shooting is done and he has seen the bird drop. Hunt it up is when I get a winger or one that went into the bush and we can't locate by sight or there are birds on the ground that I want him to flush. This gets his nose on the ground and he does a good job of covering an area.
Good luck with your pup.
If you can work on hand signals as well, that could be real "handy". I didn't do that with my dog and sometimes I wish I would have..