Paper Prototyping - Prototype your websites on paper first.

I do this all the time, I have a big newsprint pad and a whole bunch of different coloured big Crayola felt markers, and when I was designing both lovemygarden.net and my other websites, I used this pad of paper extensively. I really like the big pad of newsprint, first because it's really cheap, and second because you can just fold back a page after you've drawn on it, which gives you a nice way to organize the pages by history.

I also use it extensively for flowcharting and writing code, for doing this one complicated AJAX project, with two servers and clients that could each be in a number of states, it was really invaluable to figure out what needed to go where.