Validation and Cleanup
Once you've made the effort to convert a few documents to XHTML, it makes sense to test your documents to verify that they comply with the standard. As I've already discussed, it will probably never be good enough to simply test your document in a web browser (given the browser developers' lack of interest in 100% standards compliance). Therefore, it's probably best to start off testing with a tool such as xmllint from XMLSoft.org. To double-check, you can add a link into your page that will refer your page to the W3C's XHTML 1.0 Validation Service. To do so, simply add a link to http://validator.w3.org/check/referer. Clicking this link within your page generates a validation report on the page. Visit http://validator.w3.org to simply have their service retrieve your document on the fly.
One other tool that you should definitely check out is HTML Tidy. This handy tool will go through your HTML code and perform an XHTML-compliant cleanup. The structure enforced by XML combined with tools such as HTML Tidy and xmllint practically ensure that your code will be viewed as you intended.