Do you use Boks (http://toki-woki.net/p/Boks/) with your Blueprint CSS framework? I've found Boks to be a great way to bring some of the benefits of mockups to HTML prototyping. I still start from pen and paper first, because I find that I have to know what I'm going to put into Boks.

(I'm a developer, not a designer, but I've found myself doing more front-end work recently for myself and clients)