I work in a verrry casual office, so I also have certain advantages.
On my person...
pants and drawers for the day
In my bag...
A collared shirt, spare pants/drawers during warmer months/rainy days, lunch, and my lock, spare, flat kit, lights, and the like. All of this fits fine in my size L timbuk2 messenger.
What I leave at work...
Axe, deodorant, nicer shoes, face wipes, and some spare food (trailmix or fruit, usually)
I just have a cube for now, and i mostly go unnoticed at work, so it's not really practical or neccesary to bring in full changes of clothes, but I'll prob. shuttle loads of clothes back and forth once I get a job with a bit more 'status'. (sooner rather than later, hopefully)