To preface the following, I will state that ALL my girls KNOW the safeword and are Secure in the knowledge that using it will bring things to an Immediate halt without exception.
Also, I know them very well, their Limits (Soft & Absolute).

Humiliation/Embarrassment is something that can be as subtle as simply reminding Lhana she has a great-looking ass (she's never really been comfortable in short skirts) in public and such never fails to get a good Blush and giggle.
When going out, they all wear very high heels that lock securely around the ankle with small chromed padlocks. People see the 'flash' and immediately notice when they look down that none of the girls can remove their footwear...ad there's NO mistaking the statement made.

Yes these are 'light' methods, but make no mistake...when a good number of folks notice locked-on footwear, attention focusses and the results are impressive.
Also, such runs no risk of arrest or similar hassles.
Add in sheer blouses, no bras, and skirts short enough to guarantee they flash their asses unless they are VERY mindful of such--and such 'light' methods can deliver a massive payoff.
If your girl/girls start getting wet enough that it starts down their legs, so much the better.
Have your car seats stain/scotchguarded, same with sofas and such. After a good bit of fun in public, some women are wetter than a waterslide at peak summer hours.

It doesn't need to be Blatant, and it's mainly in the head, thus for the best results, plan your tactics around her Triggers.