How do I rotate the top-facing texture of a block?

[Flenix]

Title says it all really. I can rotate the ones on the sides, but I can't rotate the top one!

Well, I can, but I've done it in a very buggy manor and I don't like it (Stole the code from Piston, so it's technically not the top I just modified it so the fake piston can never be placed upright)

Anyone able to help me? I've spent hours trying to find a tutorial on it...

[hellokoko]

I came across this when searching because I'm in a similar situation. Anybody got any idea?