If you don't have an fmnu template (see below) and you want to change or add a command key to a menu item, you have to poke around until you find the text of the menu item's name (e.g. "Empty Trash") and then change the byte that is three bytes before the first letter of the item's name to the command key. Maybe an example would help - to use Command-T to "Empty Trash", open fmnu #1255 (ResEdit opens fmnu resources as generic resources unless you have an fmnu template, see below), and change byte number 2C to "T" or $54. Byte number 2C is three bytes before the "E" in "Empty Trash" at byte number 2F. Like most things with ResEdit, this isn't for the faint of heart, and for all I know it may have some disastrous consequences. But it works for me, and I haven't noticed any side effects.
Option-click to Hide Apps Quickly
This trick has been available in the Mac OS for years, but many people still don't know it. If you have too many windows cluttering up your screen, you can hide specific ones easily as you work. When you're in any application, hold down the Option key and click on another app's window, on the Dock, or in the Finder to switch to that other app and simultaneously hide all the windows in the previously current app.