I use a Carradice Barley bag.
http://www.peterwhitecycles.com/carradice.asp
This is a lot bigger than the miniature saddle bags many folks keep on their race bikes, but is very small compared to what the tourists are using. It does not require any sort of a rack, but bag loops on the saddle make it a lot easier. As most saddles come without loops, a small fixture attaching to the saddle rails is available.
In the bag, I keep a small pump, a glueless patch kit, two spare tubes, one spare folding tire, several CO2 cylinders, tire levers, a baggy of energy drink, several Powerbars, a "Cool-tool" type multipurpose tool, a chain breaker, a double-ended cone wrench, and +/- $20 for BigMac attacks, etc. In cold weather, I just have room to add arm and leg warmers, a wool hat, and a rain jacket.
This prepares me for almost any occurence. About the only mechanical I cannot deal with on the road is cranks or bottom bracket coming loose, and catastrophic part/frame breakage. The racer types occasionally make fun of the bag, but I occasionally bail out a racer type who didn't have enough room to pack what he needed, so I guess it works out.