This was one of the first things tought to me by my make-up teacher and friend Shawn MacEnroe (Thriller,Texas Chainsaw II, Greystoke, LostBoy's...to name a few)
Texture stamps eventually come into play but for wrinkles of all sizes you use a wire tool (I like a tool that has a wire loop on one end and a wood sort of chisel on the other).
With oil clay you simply run the wire tool across it where you want the wrinkle, cutting into the clay and leaving a groove. Then you take a medium to soft brush, dip it in acetone and brush it across the cut.
Eventually you will see that the clay is breaking down slightly and the rigid cut line is softening and turning into a rounded edge, i.e= a wrinkle.
Play with the technique until you get the hang of it but a word of caution as to Acetone and the process:
Acetone is nasty stuff and you should avoid skin contact and breathing it's fumes.
The process can be easily overdone. Take it slow and be patient to get the results you want.
Here's a good example of this technique that Shawn used for this make-up on me based on his Grandpa Make-up from ChainSaw II:
It's all Gelatin by the way.