I use the Timbuk2 Bag, really great. I have used a lot of Paniers, back packs etc. I have finished 13 months of almost daily use of the thing and it is proving to be the business. I would imagine that Chrome

http://www.singletrackworld.com/article.php?sid=962

This is a reference to the article I used to buy my bag. Cross strap which is pretty standard is essential.
I went for the large size which I think is big enough but not too small

Brian