Here are some more of my faves for the about to crack expression.
Another important point is that the expression usually says alot about the character themselves and their ticklishness.
For example, Yang there is only having a couple of fingers graze her sole and shes already having to bite down on her laughter, her eyes are wide open as she concentrates on holding it in, and her body is involuntarily twitching and shaking as the sensations rock her.
As for Mara Sov on the other pics, on her first pic shes trying to distract herself by reading a book, but the light tickling (just a couple of nails softly scratching her sole) are proving to be too much to handle and her face says it all (eyebrows raised, one eye closed, smile on her face). Her foot also cant help but scrunch its toes in an attempt to resist.
On the second pic, shes much closer to laughing but shes still trying really hard to resist. You can tell from her facial expression that shes screwed though.
Tears are already falling down her face, she has a strong blush, her brow is furrowed and eyes are closed tightly, her tickled foot is scrunched tightly, balled fists, twitching body and feet, wide smile, etc etc.
When done right, the about to crack expression is hot af because it shows the lee holding on to the last remnants of control they have. Control that you know theyre going to lose once the ler decides to take it away from them.