I leave everthing I can at work, like shoes, belt, towel, etc.

I have the Nelson Longflap by Carradice and can nearly always carry everything I need inside the bag. When there isn't enough room inside, there are straps to carry things outside the bag.

There are also frame bags that you might consider that are easy to attach/detach with velcro straps they come with. I use one when I need to carry rain gear so it won't get other things wet inside the seat bag.