1. Format your text using CSS.
2. Cascading Style Sheets (CSS) are the way to go - use one style sheet and control how text looks on your entire site. Make a change to the style sheet and your whole site is updated. It makes life a lot simpler
3. Make the font size big enough to read
4. Consider your target audience. Even if they are a group of teenage girls looking for new shoes, it's never a good idea to use tiny type. It doesn't have to be enormous, but up to a point, larger type is better. 12-pt Verdana is better than 8-pt Verdana.
5. Make the text contrast with its background.
6. The more contrast, the better. Black-on-white or white-on-black are examples of the highest contrast you can get. Use colours if you like, but if you squint at the page and your text basically vanishes, there's not enough contrast.
7. Give the lines room to breathe.
8. Don't stack lines on top of each other. Use the line-spacing directive in CSS and give it some space; I'll often set line-spacing to 140% of the height of a typical line.
9. Break text up into chunks.
10. No matter how good a writer you are, people don't want to read endless pages of text. Break it up by using headlines that reflect the subject of the paragraph(s) to follow so people can scan down to the parts that really interest them, or use bulleted lists to change the pace of the writing and slow down the scanning.

And finally (not one of the 5 Easy Ways to Improve Legibility but still quite important) check your spelling. Nothing irritates me more on a web page than spelling errors - it simply makes you look like you don't care enough to get it right. Use that ubiquitous spell-check tool.

Making your website's content more legible is easy. It doesn't take a lot of time, mainly common sense. The payoff will be text that's more readable, customers that stick around long enough to get your message, and improved credibility with your visitors.