Theres also aftermarket springs which have different weight stiffnesses depending on what you need. I run MGT copper springs and Maxx orange depending on the driving conditions.
You can change the springs like phins fan said, you can also replace the shock oil to a thicker oil, I like to use 60 weight shock oil.
Also, the position you put the shocks in has a big affect on what you want it to do, its the pivot point that you change when the shocks are placed going strait up and down or if they are on a angle, the stiffness will change when you change the pivot of the a arms.