At the bottom of the page (after the cut), my checklist in categories. The main category is for things that go in my bag, broken up into subcategories such as clothes and toiletries. I also included the "on my person" and "on my bike" categories because I have actually managed to leave these things at home. In fact, I've forgotten most of the things on this list at one point or another since I started bike commuting (going back to Cleveland). Anyway, I started this list a couple of weeks ago, and I've used it several times since to good effect--no wardrobe or bike mishaps in that time.
Amazingly, everything fits into my Brompton O bag, even when I bring extra items like a bag of coffee beans (though I did put extenders on the front straps to enable carrying more bulk).
|Bike and O bag tucked into a corner in the locker room|
(ignore the black backpack on the right, that's not mine)