I don't know about you, but I sometimes have trouble "seeing" my characters vividly if I don't have a pretty good grip on their physical appearance. They tend to be sort of vague and faceless as I write, unless I "cast" them. This may not work for everyone, but something that helps me is to have a particular actor/actress in mind for the "role" I'm writing.
Before even starting my book, during the characterization process, I "cast" my main characters, figuring out which actor/actress would most physically fit the description in my head. That way, their features are stronger when I write because my brain is already familiar with that image. For example, the two main characters (Brooke & Adam) in one of my novels, are "cast" as Reece Witherspoon and a Welsh actor named Ioan Grufudd.
Another trick I use to help me picture things more vividly - and I know this sounds a little extreme - is to create a screensaver on my computer that's got the character pictures, and places for the setting (in this case, a village in the Cotswolds).
This, for me, creates vivid images in my mind that I'm able to (hopefully) transfer to the page. Because if I can see it, I can write it. Plus, it's fun! I love creating this screensaver during the brainstorming process because it gets me involved in the story/setting/characters early on. Anybody else use these tricks? Or others? I'd love to hear about them...