Do use a consistent look and feel
Your site should stand out as a whole. Use the same look and feel for all the 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 the 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 the 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 the 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 the 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 the cells to become too large. Thus making horizontal scrolling necessary. So limit the number and size of pictures, long words (e.g. long links), predefined text etc.
Don't overuse frames
Use only a limited number of frames. Always check if the screen doesn't get crammed if a low resolution screen is used.
Do vertical align the content of table cells
You do you use table cells to get your info on the right position? Then be sure to align the content vertically.
Don't mix horizontal aligning
Mixing centered text and text that's left aligned makes a mess of your page. If you want to center text do it consistently. Exceptions are centering text of headers or centered text placed in a border.