I my opinion, what you carry with you depends on 2 things:
1) where you are travelling (i.e. how far you will be from a cycle shop at any one time.
2) how much you want to be prepared for any circumstances.
My commute is only 3 miles, if i take the direct route. but by this time I'm just getting warmed up and am dissapointed on a nice day that I have arrived. So i have extended the route to 12 miles each way. I don't take a spare tube, just a few self doing patches. as where I go, you're never more than a few miles from a bike shop.
Colthes wise. I take a ruksack, in it I have spare underwear (a must when it is raining), trousers, shirt, tie & lunch. I leave shoes, jacket & "freshening up" products at work.
We don't have a shower at work (more's the pity) so I use those baby wipe type things... they're really very good.
Leave yourself plenty of time each day for commuting by bike. it take a lot longer for several reasons.
1) you have to eat before you leave... otherwise you won't make it!
2) you're slower than a car
3) if you get a puncture you don't want to have to use that as an excuse for your tardiness.
4) it will take you a while to cool down and get presentable after cycling in that distance.
I leave with about 30 minutes extra time than I "ought to need". 99 times out of 100 i will arrive 30 minutes early, but I'd rather that than be late.
best bit of advice i've had is this:
"Enjoy it, it's not a race"
