The most effective training tool you have whenever training your German Shepherd to "Wait" is simply
the tone of your voice. Your voice during training is one of the most effective tools that you can use in any aspect of dog training.
Your GSD always looks to you for approval and direction and your German Shepherd needs to be able to understand your vocal tone to properly respond to it. Keep in mind that your dog is a pack animal and that you are his pack leader.
Let's delve a little deeper into it. Whenever your German Shepherd is doing anything at all, try saying anything in a moderately disapproving tone. I imagine your dog will probably stop whatever it is doing pretty fast.
Try this for example, if you want your dog to wait for you at the door before going outside, say "Wait" in a serious but not an angry tone, just before he gets there - your dog will probably pause... waiting to see what you want next. If he continues on without pausing, say "NO, WAIT!" in a firmer tone.
Basically, the command "Wait" means for your dog to momentarily pause.
In the above example, teaching your dog to wait at the door gives your GSD the joy of anticipation - waiting on what's coming next. It lets your German Shepherd puppy know that if he pauses just for a few moments, that he can soon continue outside to the next adventure with your full and complete approval.
One of the best uses for the command "Wait" is when you are putting your dogs food down.