I don't know if you realized that you were indirectly asking for everybody's favorite trick, but here's mine:

I use Quicksilver hotkey triggers to bind my most commonly used apps to cmd-F1–F6 (that's as far as I can easily reach with my left hand, but to give me more spots, I stack them up by using cmd-opt-Fkeys also). Since the same action handles both launching and switching to, I'm never more than 1 "point" from most of the apps I need, whether it's already running or not, and it doesn't matter if my right hand is on the keyboard or the mouse.

It turns out to be much faster than cmd-tab, because you don't have to pause, interpret the order of the icons, count how many presses you need, and then execute those presses. It's just 1 movement, and BOOM, you're in Mail, no pausing or thinking required.

I'm sure you could do this with other tools too, and the quicksilver implementation detail isn't the important part, it's the instant-access that you come to rely on and use without thinking.