For some reason they won’t let you use the Javascript data attributes with a custom button so you have to add them to the URL in your HTML. So for your example:
You would then add &text=your%20text with your text.
<a href="http://twitter.com/share?url=http%3A%2F%2Fdev.twitter.com%2Fpages%2Ftweet-button&text=my%20text%20here" target="_blank">
The other codes are the same, just add &related= etc.
Make sure you replace any spaces with %20 and that is it.