A fairly important usability tip for web design is to make sure you include all the important details to support your content. A couple examples would be to make sure table columns have labeled headings, and all graph data is labeled.This may seem to be an incredibly obvious tip, but even the big boys make these kind of mistakes.
Example of Not Double Checking
Here is an example of Yahoo not labeling the comic on the page specifically designed to show it. Additionally, though it is not seen in the screenshot, the title of the page does not reflect the comic name either. Not one clue to the name of this comic.