There are two different ways of reversing a lens, both of which have been mentioned in this thread. Each one requires a different bit of hardware to do it. One way is to reverse a lens onto a camera body or extension tubes. This requires a reverse adapter that has on one side, the male mount for whatever camera system you're shooting. The othe side of this adapter has a male filter thread matching, or adapted to fit, the filter diameter of the lens you're reversing. Focal lengths of 50mm or less are what you're looking for. Primes work nicely (and for 50s f1.7, 1.8 and 2 are reported to be better because of flatter fields than the faster f1.4s and 1.2s). I've actually had good results from reversed kit zoom lens too, which offer variable magnification by zooming, with higher power being achieved with shorter focal lengths.
The other technique involes reversing a shorter focal length lens onto a longer lens already on the camera body, like a 50 reversed on a 100mm lens, which would yield 2x magnification (100mm divided by 50mm = 2, if you want to see the math). The adapter ring used for lens coupling like this has male threads at each end of the appropriate filter diameters for the two lenses being coupled. I've made my own coupling rings using combinations of stepping rings epoxied together.
And while I'm at it here, I'll post a couple of my favourite links that are great resources for macro techniques and ideas.
From right here on the Forums
CHEAP MACRO -- Buying or exploiting a lens for ultraclose work - PentaxForums.com Extreme Macro Photography
More specifically about the use of Raynox lenses (which I have not yet added to my tool kit, but are on the list) which many Forum members have used with excellent results:
The Raynox Macro club - PentaxForums.com