I've found that really soft skis like my Nordica Supercharger's will automatically start rockering on their own due to a lot of butters, tip stands, and nosepresses. I think that most skis that are really soft will rocker simply by putting them in a vice with a book or piece of wood placed between them.
I've also rockered an old pair of K2 Enemies, which is a really stiff ski, and the key there is to use heat. I put a clamp on my skis at the points i wanted my ski to rocker and shoved a book in there to hold it in place and then applied heat to the area for 5-10minutes on each ski about 3 times at each rocker point. I used an embossing gun that mom had which outputs heat at about 650 degrees. I kept the gun about 3 inches away from each ski.
Here's a look at the setup:
Theres a clamp at the tails where i just heated it with a book holding the form of the rocker, and at the tip i have some books holding the form inbetween heat sessions.