I wished Mame would have a hotkey which could be pressed during gameplay and show a JPG file. The JPG file could be a picture of your control panel with text describing each buttons.
Actually, you can do this BEFORE the game starts with most front ends.
You just need to build the picture files and save them in a new folder (titled instructions, or some such).
You can then set the front end to display that information in a box, just like the screenshots.
That doesn't help in game, but it is a really good method.
I haven't gotten around to making any yet, but it's planned for the future.
Another thing that is helpful is to have different colored buttons.
That way you can tell people to "hit the orange button" for a particular function.
That works really well with my 5-yr-old nephew when he's playing Tempest.
He knows that the red button shoots, and the orange button "erases everything".
I've got two 4-button modules for mine.
One is the red/green/blue/yellow like Neo-Geo, and the other is blue/yellow/red/white like mousetrap.
That helps simplify instructions.
I also map as many buttons as possible to a function within the game.
For games like Donkey Kong, I map ALL the buttons to jump, so that it really doesn't matter WHICH button gets pushed.