The TextMate HTML bundle has a lot of helpful commands which create or work with tags.
Starting with XHTML, tags with an an empty content model like `<hr>` should be written as `<hr />`. As to whether or not one should adopt XHTML, I’ll just link to [Ian Hickson advocacy against XHTML].
To make TextMate use XHTML tags, go to _Preferences → Advanced → Shell Variables_.
Here add a new variable with a name of `TM_XHTML` and a value of ` /`. That is a space followed by `/`. This text will be inserted verbatim after each tag name with empty content model.
Additionally the presence of the variable will also enable XHTML mode for Tidy, Markdown conversion, etc.
[Ian Hickson advocacy against XHTML]: http://www.hixie.ch/advocacy/xhtml