Webdesign tips

I’m about to design a site for a european math project. It will be a demanding job for me, and it is important that the design works well. I will not be programming the site, only designing it. So I wonder if anyone has some good tips, some do[size=1]s[/size] and don’t[size=1]s [size=2]and/or something else usefull.

The site needs to be:

  • Easy to navigate
  • Clean and fresh
  • Professional, no “cool urban” stuff

basicly I’m looking for things NOT to do, but any tips are welcome.