When teaching commands... in the beginning.... do voice + hand gesture commands simultanouesly... and heavy treating of course. Then.. when they've got the command good... you do just voice.... treats of course... then you do just hand gestures...treats of course... and Always with the Praise! Then when they're good with both... you start weening off the treats 90/10 80/20 70/30 and so on. And won't take long at all they'll execute just for the praise and "fun" of it. Also you can do voice + head nods in order to teach them head nods to execute commands... same process.
Very important... ONLY TREAT when they've executed the command EXACTLY as you'd like them to! They WILL try to "short-change" you on their execution. Do not allow them to. You don't do it right? You don't get the treat.
Another common thing you see you whip out a treat, and they'll bust out and execute all the commands they know all at one time one right after the other. When they do that you gotta say "No, I didn't tell you to do all that." Then take a step backwards, and when you've got their attention again with a "Ready?"... execute your command again.
Develop a "No!" sound that you will use for immediate reprimands. Both a loud version and a soft version. Such as "AHHHHT!"
Develop a specific sound of whistle. So that way they know it separate and apart from anybody elses whistle to their dog.
When doing your "Come!" command outside... when they are some distance away... in the beginning always precede it by your particular whistle, then the "Come!" command.
And a particularly useful trick I discovered with my first dog... if your dog gets easily distracted when returning back with the fetch... Hop a little from side-to-side... first a little this way.... then a little that way... make it random... it gives them change in your position that they naturally want to focus on, which prevents them from getting distracted during the return of the fetch over a longer distance. Of course when they arrive at your position... treat and lotsa praise in high-pitched praise voice! Eventually just the praise is all that's needed.
Big thing is just huge amounts of repetition.. and YOU being consistent in what you demand from them. Most folks give-up and cave-in during the training... or... they have a spouse/partner who does behaviors that EFF up and undo the training you're doing with them.