Just practice at it, then one day it will just happen. I ride 46x16. My first time on a fixed bike I was skidding right away with no problems, not very impressive skids but i was doing it. When I was young all my bikes before the age of 15 were fixed gears, seemed like that's how they made kids bikes in the 70's. So when I had to skid I obviously was doing the same thing people do now on fixed gears. I don't know call it muscle memory.

I am very impressed seeing people skid effortlessly seated, especially going down a hill and skidding every few feet alternating sides. A little tip is whatever foot you are going to use for the back pressure, as soon as that foot is on top of your rotation start immediately to stiffen your leg and pull up with your other foot, really its all about timing and different muscle usage. As you practice more you will work those certain muscles in your legs you never used before riding fixed.