D'pends what you need to carry. A rack trunk bag works for me - I am able to fit my work clothes (khakis and a polo shirt) and my lunch in the main compartment, one side pocket holds my spare tube, patch kit, levers, multi-tool, rag, spare blinky batteries and both bag & saddle rain covers, the other is for my wallet, cell phone and keys. My rain cape ties down nicely to the top and I have a PB Superflash mounted on it to augment my rack-mounted 4D Toplight. Despite the rain covers, I still put my clothes and lunch into plastic grocery bags...rain is like a really mofugly woman - one layer of protection is not enough.

I use the Topeak racks and bags...with multiple bikes it's nice to be able to have just one or two bags (and even a basket) that I can just slap on most of them.

I also have like 3 sets of panniers which rarely get used anymore since I got my Big Dummy....but they used to come in handy when I needed more room than the rack trunk bag could handle. Again, even though the panniers were allegedly water-resistant/proof, I packed my stuff in plastic grocery bags.

Your lock should stay at work, as should your shoes, belt, ties (if applicable) and a full set of spare clothing (Murphy defeats the best laid plans sometimes), as well as a wash cloth, towel, deodorant and any other toiletries you may need.