One thing that will help is developing a solid hold. How I trained my dog when she was a pup through a bird training class I took with her sums up to this:
Sit down on a chair with the dog in front of you.
Present a bumper to them and say "fetch" (or whatever your command is).
When they take the bumper, give a release command and then give them a treat.
Eventually build up to when you make them take the bumper from you to holding the bumper for a few seconds. I would say "hold" after giving the fetch command. Sometimes rubbing their neck or head while saying "hold" will keep them holding onto the bumper longer.
Make them hold it without mouthing the bumper. You want a solid hold, no mouthing. If they start mouthing it, say "no" and take it from them and do it over. When they successfully hold the bumper for a few seconds without messing with it, give them a treat and praise em up. After awhile you drop the "hold" command and you just say fetch but they should be still giving you a good hold.
Eventually they'll learn "fetch" means take the bumper and hold it until you say give.
That's just some general cliff notes of what worked for me.