Get a carradice bag and a Bagman from Wiggle. Also get one of those bungee nets. When you don't have a whole lot of stuff, just stuff in the bag; when things overfill, stuff as much as you can in the bag, hook net onto bagman towards your saddle, stack more stuff on top of the bag, run net over stuff, and finally latch onto far end of Bagman. I use a Barley, and despite being the smallest bag in their lineup I can carry shoes/change of clothes/lunch with ease. The Barley is small enough that I usually leave it on all the time w/ my repair kit and a couple candy bars in case I forget to bring something to eat.