Sarah Baker says:

That can be a tough one! Sometimes I will try and create a hold and carry by tossing the retrieve item. Sometimes I will keep ahold of the item and ask for more than one “take” before rewarding. Multiple takes can turn into a grab. You can teach a chin rest in your hand, then add a take to it. You can also see if watching another dog do it helps. That was the break through for Skeptic. Be patient and use high value rewards!