disclaimer: the following works for me, because i work where in an office where i can store my bike in the copy room (so i'm lucky enough to be able to leave my locks at home). i also have a gym membership which is near the office. finally, there is a great supermarket just a few blocks from my job. i'm very aware and grateful that the confluence of factors allows me to ride fairly easily to work.
that being said, i don't understand why so many commuters carry so much stuff with them, e.g., lunch, shoes, etc. i did at first and found a little planning helped me out.
i work in an office where i have to wear a shirt and tie...and suit a couple of days a week. i have a rear bike rack where i carry a neatly folded pair of slacks, shirt, etc. i wear a timbuk2 medium messenger bag, which i find to be infinitely more comfortable than a backpack, where i carry my wallet, cell phone, cash, etc. inside. i store 2 pairs of shoes under my desk and keep a handful of mostly neutral ties in my drawer. i'm no great style maverick, so i keep a basic blue or grey suit at the office (occassionally switching them out) i hang a towel on a hanger in the file closet.
i occassionally pack lunch for a few days early in the week, e.g. sandwich meat, bread, containers of soup, bags of salad, oatmeal, peanut butter (not all at once). i also will go to the supermarket and buy food and keep it in the fridge. again, i'm lucky enough to have a fridge at the office where i can store food.
granted my situation is ideal, but a little planning can help lighten the load.