At work I keep a pair of jeans that I use for a week -- shuttle in on Monday, out on Friday, and a pair of shoes for use when I ride my caddy with the clipless pedals. I also keep a polo shirt and pair of socks there in case it rains on the way in or something along those lines.
As for what I bring with me, it depends, but there's usually a lunch in there, sometimes a laptop and a couple of books. I always have my wallet and cell phone, along with a spare tube, some tools, either a pump or some CO2, my bus pass, and my work ID. Other common tagalongs include a paper or two that I'm reading at work, some water, miscellaneous office junk, a map of ABQ and its bike trails and lanes, a jacket or wind vest, lenses for the sunglasses, and a pack of Starburst. If I'm using the messenger bag I usually go lighter than I do with the pannier.
What do I have two of? Nothing. Though I keep meaning to bring some spare tubes into work as a JIC.
How do I organize my pannier? Wallet and phone in the pocket, bus pass clipped in my left shorts pocket, work ID clipped in the right, everything else just kind of dumped in there with the harder stuff on the bottom and the squishier stuff up top. I'm pretty sure organize is the wrong word for it.