Visually attractive websites

I recently posted a question and gary.turner gave me some code. This is part of it:

.desc img {
      border: 1px solid black; /*for clarity*/
      margin-right: 1em;
      float: left;
      }

He is right. The solid black line simply works on the page.
So, it is one thing to know how to write good css, it is another to make visually good websites.

My question therefore is: what are the guidelines/policies YOU use every time, when making a website. And I am not talking about how to write your code. Just the visual side of it.

Thanks beforehand.