Here are a few things to consider when setting out to build a site. Do not limit these principles to web sites, they also apply to any kind of visual design.
Simplicity
Your design should make simple, common tasks simple to do. Dont forget the original purpose of the site and dont detract from that by adding unnecessary clutter.
Structure
Your design should organize the user interface in ways that are apparent and recognizable to users, putting related things together and separating unrelated things.
Visibility
Good designs dont overwhelm users with too many alternatives or confuse them with unneeded information.
Reuse
Always reuse internal styles and colors. Dont deviate from the styles you establish. The reuse principal eliminates the need for users to rethink, and it makes a website easier to navigate.