I commute to work on my road bike, which has no rack mountings. used to use a rucksack, but I have found that the messenger-style bags are much more comfortable, as the load sits much lower down. having a waist strap stops the bag swinging about, and helps to support the weight a bit too.
I have a hard-tail mountain bike which I used to go touring round Europe last year. This also has no rack mounts, and so I purchased a seatpost mounted rack, with side supports. I would not reccommend this to anyone, I found it to be quite dangerous. On a number of occasions, on bumpy surfaces, my panniers swung round and snagged on the wheels. I did not come off, but my panniers did get thrown off and across the road, breaking my digital camera in the process.
I don't think I can use p-clips, as I have a wishbone-type seatstay. Any ideas what I could do to fit a proper rack to this bike? I would love to go touring on it again, as it is really light and comfortable