what i do with my gundogs mate is teach them the hold command. Sit them down and place the ball/dummy in their mouth and repeat hold hold...... etc... and then a release command - i always use 'leave that' but whatever works for you. after a few times and you can see the dog is understanding the command you can just say hold once and then wait 30 seconds and then the release command. when you are sure she knows the hold command say hold as she is coming into you with the retrieve, you should find she will hold it out of habit and you'll have the perfect retrieve, if she drops it place it in