NotFred
I have a 15 mile commute (each way) - here is my routine and what I carry on the bike:
The bike has a small bag under the seat which contains spare tube, patch kit, tire irons, multi-tool and a $20 bill. There is a frame pump and a lock which are attached to the bike - I never think about these things, they are always there in case I need them, which I rarely do.
I can't stand having something on my back when I ride, so I carry a pannier which has the other stuff in it - today it contained:
Clothes for work, rain gear (forcast is for rain this afternoon), lunch, cell phone, wallet, 2 apples.
At work I keep: full set of toiletries, 2 towels (I use one on me, one on the floor, each week I bring one home and wash it, and rotate), stuff to make breakfast (oatmeal and bran muffins) - from time to time I have to carry stuff in to replentish my stocks. I also keep a pair of shoes at work. I also keep a spare set of clothes at work for those days when I arrive and say 'Damn, I could have sworn I put a pair of pants in this pannier'.
When I arrive, I turn on the computer, check for anything urgent, and give my body a few minutes to cool down, I then shower, get dressed and fix my breakfast. I am fortunate in that the bike can stay in the office with me - on the days when I do drive, I now get razzed by my fellow workers when the bike is not there and they will call me a wimp.
After work I put the work clothes in the pannier, ride home, and usually have a quick shower before getting dressed in something casual.