I was just kidding about my wife's weight. She's 130: and I'm 225. But to answer your question, you twist the bottom half of the shock to increase or decrese the preload. It takes either a spanner wrench or a strap wrench. There's numbers near the bottom that correspond to the preload. Higher=stiffer.
I tried 4 for a while but it was too jarring. I keep mine at 3. You amy also want to consider nylon shock bushings, whihc will stiffen the ride and give better conrol. The rubber stock ones don't last much more than 6-7000 miles before they get shot.
P