Well I can tell ya what I do.
On the front I squeeze above the knee the cap. You are really trying to get the lower part of the thigh muscle I think. You can also scratch above the knee cap (lower part of thigh muscle) too.
On the back I scratch in the middle right behind the knee. On the back I also put pressure on the two tendons of the back of the knee. There is one on each side. I run my finger over it with pressure. Just act like you are tickling somebody's ribs, the only difference is that there is just one "rib" to run you finger over instead of many stacked together. You can also squeeze it too, but it can't be the same squeeze you do for somebody's tummy. When you "squeeze" it you are really just using your finger and thumb to move over the tendon, just like with tickling ribs.
Also, I find that scratching (on the front or back) works better if the lee has some kind of pants on. The pressure stuff works better directly on the skin.
All this stuff works on Meka. None of it might work for you because as Sammi said "it just varies from person to person", but good luck! haha