Just put the URL between angle brackets, i.e., like this:
<http://www.somesite.com/a/really/long/url>
When formed in this way, the link remains clickable for the recipient of the Mail message.
[robg adds: This is a relatively basic tip, and one that I've personally known for a long time, though I don't know where I learned it. In searching both the site and Mail's help, I didn't find this documented in either place, so here it is.]

