Basically, make sure your markup is semantic.

ie. tables for tabular data, lists for lists, paragraphs for paragraphs and so on.

Tables used for layout make your code bloated, they cause accessibility issues, issues with handheld devices and so on.

Separate your presentation and markup. Use (X)HTML and CSS.

If you have issues with CSS, or simply want to use a nice framework (I am using it for a few sites and very content with it) try out blueprintcss - Google Code