How did you teach Spencer the "gentle beak" thing?
Trial and error! :D Unfortunately, the learning process did involve a lot of getting bitten, and even now it doesn't work all the time (like if he's feeling particularly food aggressive or if he gets carried away while playing). But it's cut down a LOT of the amount of biting he does, which is important because green cheeks have a tendency to be kind of nippy, especially as adolescents.
I started out by gently pulling my hand away and saying, "Gentle beak," when he would start biting too hard. At this point he clearly didn't understand the command, since I had just started using it, but he would always pause and look at me, and I would wait a minute before giving my hand back to him. Usually he seemed to get the idea and would play more gently. Over time as he got better at adjusting his beak pressure in response to the command, I implemented the "three strikes" rule: he got a maximum of three "gentle beak" warnings, and then the next time he bit too hard, he got a five-minute time-out, during which he not only went into his cage, but got totally ignored. If he bit me when I went to let him out, he got another five minute time-out. Since Spencer LOVES attention, the time-outs were very quickly effective.
As we developed our relationship and he let me start petting his beak, I started doing what I think was the most helpful thing: I would pet his beak gently while saying in a praising tone, "Gentle beak," accompanied by other praise (e. g., "What a good bird, nice bird, who's a sweet bird," etc.). This way he came to associate "gentle beak" with gentle beak pressure and mellow, cuddly-type interactions.
Now, I would say the "gentle beak" command works about 85% of the time, and I rarely have to put him on time out for biting anymore. I've been able to get him to let up in the middle of a bite by saying, "Gentle beak," and then gently petting his beak, at which point he usually starts doing the gentle "mutual beaking" (where I gently pet his beak by holding it between my thumb and forefinger while he gently beaks my fingers) thing instead of biting.
I would say one of the most important components of getting the "gentle beak" thing to work, though, is that I pay attention to Spencer's body language so I know to back off BEFORE he feels like he has to resort to biting. Biting usually is the result of a bird trying to convey, "No, I don't want to do that," in the only language he has available to him (posturing, angry/irritated vocalizations, retreating, etc.), and feeling like those warnings have been ignored. In order to cut down on the amount of biting, therefore, it's important for the bird to know he can trust you to respect his boundaries BEFORE the point at which he has to resort to biting. A bird whose boundaries are ignored too often can start skipping over the "warning" body language and just going straight to biting.
Obviously there are times when I have to get Spencer to do things he doesn't want to do, like if I need to move him somewhere and he doesn't want to go, or if I need to towel him to clip his claws, and these are the times I still tend to get bitten, but in this case it's important for me to continue to have him step up even if he bites so he understands that he can't control my behavior by biting. I know this seems contradictory, and it IS kind of complicated, so I guess to summarize: respect your bird's boundaries as much as you can, and if you have to get him to do something he doesn't want to do, don't let his biting affect your behavior.
I hope this helps! It took the better part of about four months to get Spencer to really respond to "gentle beak", so patience and the willingness to let yourself get bitten a few times are necessary. :D Let me know if you have other questions!