I used to ride to work every day. I'd suggest not wearing a pack. It's hot and it throws your balance off. Get a rack and a rack bag and/or paniers.

I strongly second the suggestion of carrying a spare tube rolled up in a baggie with talcum powder. It saves a ton of time. The talcum powder makes it tremendously easier to get the tire back on. A good frame pump is a must. And a patch kit, for when you get your second flat.

Also carry any screwdrivers, wrenches, etc. that your bike needs. I never carried a multitool. They were heavier than the individual tools. You'll definitely need composite tire irons (the metal ones suck). I found 2 to be enough.

I carried a mini FAK with some bandaids, Neosporine and a bandana. Cuts and scrapes will be common. Carry some quarters, a spare $20, a credit card, and ID.

Someone mentioned lights. I found those flashing LED tail lights to be the bomb. They help drivers see you, which is safety rule #1. I never did find a really good headlight. Hopefully technology has improved.

I also did the deal where I brought in a week's worth of clothes and kept them in my filing cabinet. It was really nice when my wife worked at the same company. She could carry the heavy stuff in the car.

Kevin B.