I just read the updated version of Don't Make Me Think by Steve Krug, the classic web design book. The book itself is designed according to his principles and has titles and subtitles that summarize the text so you can skim it in a few hours.
My top 5 takeaways:
1) If you have to think about what to do on a web page, there's something wrong.
You should be able to navigate a website without doing any cognitive thinking.
2) You probably scan websites, very few people read text.
Experienced web users just scan a page and notice a few highlighted things. They will start clicking and use the back button if they don't find what they want.
3) The use of common navigation conventions helps a lot.
Just like you recognize a yield sign or a stop sign when driving, you'll recognize common web conventions. Logo in upper left, consistent tabs at the top, standardized search boxes, and button labels that are simple and direct.
4) On every page of your site, make it clear what to do.
Keep a common structure for every page, no matter how deep in the hierarchy. Make sure that someone will know: a) where they are, b) what to do.
5) Do a little usability testing, save a lot of pain.
Do quick cycles of video-taped usability testing. Even if you just invite 1 or 2 friends to test, you will quickly identify your biggest navigational quirks and annoyances.
It occurred to me that these are good guidelines for any visual communication, even email. I'm going to try writing some emails that are designed so that the recipient doesn't have to think.