Do use a consistent look and feel Your site should stand out as a whole. Use same look and feel for all pages at your site. This way your visitors have a sense of recognition when they visit various pages. Using stylesheets makes it much easier to maintain look and feel of numerous pages.
Do use recurring visual elements Repeat visual elements (images, colors, fonts etc.) on several pages. This will add to a consistent look and feel.
Don't use dark backgrounds Dark backgrounds tend to make text less readable. So avoid dark colors or dark backgroundimages. If you do need them, use a nonserif font for text (like Arial, Universe, Helvetica) and be sure to not to use a small fontsize.
Don't cram your pages A page with text pushed aside against border of a table - or an image - looks awful. Don't cram your pages, use colspan and borderspan for tables and vspan and hspan for applets and images.
Don't push your table out of screen Tables are very flexible. They're able to get almost anything more or less visible on a screen. But by putting large elements in a table cell you might force cells to become too large. Thus making horizontal scrolling necessary. So limit number and size of pictures, long words (e.g. long links), predefined text etc.