Desert camo, heh
I use a courier-style bag that sits flat on my back and has an over-the-shoulder strap. It just happened to be my briefcase/laptop case before I got the bike, so I don't have a ton of experience with other bags. The nice part about the flat bag is that it doesn't flop around at all when you ride; I sometimes forget I have it on. It's also got a nice little web pocket for a water bottle.
I guess if you have a lot of hills, you should throw down a little scratch and invest in a
Camelbak, which is the incomprable best for keeping yourself hydrated; a critical factor of cycling up hills in the heat...
Buy yourself a real light poncho-style jacket would be my recommendation, to save space and weight no matter what ya go with. There are even some nice designer ones available which will fold up into practically nothing and have a clean line to them if you want to go that route.
The really nice thing about cycling is that it is a workout that allows you progress at your own pace and desire; if I want to get to work the fastest and easiest way, I can, and when I feel like working more I just add some scenic detours into the route. I've discovered some beautiful parts of my town that I never knew existed which is a real plus.
Cheers
Cycloptichorn