I thought it would be interesting to hear some. I will post mine, and all were given by professionals actually successfully showing. BTW, if you agree with the any statement below, please post your thoughts. May be I'm wrong (I have no problem to admit it).
1) Standing martingale (I already discussed that before). The only way to go to have a nice head set. No-no for running martingale.
2) Running martingale - brings the horse from forehand and makes work from behind (still can't figure out how THAT can be accomplished especially if the horse keeps head low, I always rely on transitions ).
3) Double jointed snaffle is very harsh and don't even argue with it. The only way to go is single jointed loose ring happy mouth (because, you know, it's the only one worked for my girl's poneh)! (I loved that one, BTW )
4) Thinline pad just can NOT work - it's too thin! (so get 3 of my shims to stick on top of your normal pad for $100/each, came from same source as #3).
And of course endless statements from the beginner riders (who don't know how to ride but got horses) about beauty and gentleness of the mechanical hackamore (because when you pull on it when you horse bolts, it doesn't yank on horse's mouth! True statement from one of the rider).