Put a single digit before the label name. For instance, make a label called "1 - Important Stuff". You can then make another called "2 - Family Stuff".  Now, when combined with the "." key that Google offers to bring up the label list, you can quickly put items in to various label buckets.  You have to hit "enter" after you're done with it, so you'll be hitting three keys. If you have a standard numeric keypad this technique will be very fast as the keys are all close together. I find this technique much faster than using arrow keys or the first letters to find what I'm looking for in a large, ever expanding list of labels. It's definitely faster than the mouse.

Now all you have to press to add an item to "1 - Important Stuff" is "." then "1" then "enter" and it'll be done. Simple, yes/no?

I have a few items that are always the same and a couple of others that I change as particular topics come and go. Renaming labels is easy to accomplish this. It can be found on the labels page within your account settings.

That's all there is to it. You won't even need to install any plugins or Greasemonkey scripts to use this technique, so it will work regardless of what machine you use, just like Gmail itself. Do you have a Gmail tip to share?