just send it to them and write a few lines?

As with most pitches, try to keep the writing to a minimum and get straight to the point. I'd say a sentence introducing your brand, what you are offering out of the pitch and what you expect in return. From what I have learned and have been told is to keep it as far as ambiguous as you can get. Also, all pitching articles are good articles if it pertains about pitching - you just make the small changes to fit your product. Just google "Pitching a Product" or "Pitching an Idea" and read up on all the articles you can. The more you know, the better.

short, sweet, to the point, using active language

Godmachine had a cool one on his blog, and the J fin book threads not dead has some literature on the subject
http://godmachinewip.blogspot.com/2012/04/how-to-get-clients-emailing-them.html
http://threadsnotdead.com/

and Bobby Chiu isn't specifically for shirts, but freaking kick ass.
http://theperfectbait.com/about.php

The short versions is just maks shit till it's not shit anymore post it anywhere you can find eyeballs, and people will find you.

and if someone dosen't respect you enough to pay your asking rate, they are probably going to not respect you on a lot of other things, just let it go.