I keep two types of Bag, although in the near future I'll be looking for a slightly more discreet bag.
For just a day trip out somewhere, I'll normally take just the one body / lens in a smaller bag (normally the Lowepro Nova 160 AW, which you can currently get on Amazon for £34)
Or for events / festivals and other stuff I tend to use a backpack, these are a little more expensive (normally the Lowepro Fastpack 350 which is currently around £80 on Amazon) but it gives space for multiple lenses, spare body, flash, laptop and loads of other bits of kit, if you need to move a lot around, my only regret is not going for the AW version...
As I say though I'm looking at a slightly more discreet bag, perhaps for travelling, and I'm also trying to not just be a Lowepro fanboy xD