Many GTD solutions force you to spend more time categorizing etc then I feel is worth it (in many cases) So I have a simple system:

I set up saved searches for my most frequent categories: Calls, visits, buy, email etc.

When I input I always make sure that the appropriate word is incorporated in the phrase.

For example "Call Bob Tomorrow"

I know that I can go to the saved search "Calls" and will find "call Bob". simple? yes time saving yes!