How do you actually pick a comfortable saddle?

Obligatory courier chip in, saddle comfort is even more imporatant when you've got 15kg in your bag ;)

I probably weigh more than you with the bag.. it's your bony arse that's the problem.
Get a nice office job where you don't move and fight off the boredom by comfort eating and you'll have a fine padded arse that will suit most saddles. :)