If you find yourself reaching for the mouse too often, Web app ShortcutFoo (a free demo with feature limitations, and a $9 paid-for app) can help you start using the keyboard for everything.
That's right. Not just in Gmail, but in Photoshop, Microsoft Visual Studio, Eclipse, and more. So, put your mouse out of reach, and let your fingers do the talking. Unlike KeyRocket, a shortcut learning application that teaches you while you work, ShortcutFoo works as a dedicated learning environment.
You go to the website to learn. Keyboard shortcuts for every application are organized into bundles of related functions, each with just a handful of keys to master. Shortcuts are all about muscle memory, and ShortcutFoo's simple learning interface reflects this. A function description flashes on the screen (for example, "Select read conversations"), and you must tap the right key or key combination for it. A new description immediately flashes on the screen, awaiting another keystroke. A running log of your taps shows whether or not you got each shortcut right.
ShortcutFoo offers two learning modes, Practice and Drill. See all software downloads.
Practice mode is for learning shortcuts you don't know yet. It prompts you for the shortcut, but if you wait a moment without typing anything, it then reveals the shortcut and waits for you to tap it. You can also set Practice mode to reveal the shortcut keys instantly, or not show them at all. A Practice session lasts as long as you want it to, and you get accuracy stats when you're done.
Drill mode is for when you already know the keys but need some help committing them to muscle memory. Unlike Practice mode, Drill mode offers no helpful hints. Another difference is that drills are timed. You have a minute to show your skills, and when the time is up, you get a summary showing how you did. You can change the default one-minute duration.
ShortcutFoo is very responsive and fast, and its interface is free of advertising, pop-ups, and other annoyances. The free version doesn't require a user account, and lets you practice several bundles from every editor--plenty of functionality for most users.
If you wish to dig deeper, a paid account costs $9 and will also open up all of the bundles. ShortcutFoo's paid version also lets you add new shortcuts and new applications, so you can use it to learn any program.