With most restaurants closing down for a bit I am assuming this is something you are working towards? What do you currently perform and what resources do you currently have? But Some good accessible and low cost sources off the top of my head...
Kostya Kimlat and Karl Hein both have penguin lectures that are 8ish dollars each and both have some excellent information about working restaurants. A mod here, Michael O’Brien, has a YouTube channel where he discusses this type of stuff. That would probably be a good place to start.
But just a thought, in addition to any tricks you already know do you have tricks for things people sometimes do when dining out like; birthdays, anniversaries, dates etc?