What is the best reaction to have when a woman keeps staring at your ass?

A joke is the best policy.

One of the women that keeps doing it is very pretty. Lol. Anyway, what sort of joke would you recommend?

It depends on certain aspects.

The place.
The intention.
Your personality (what seems reasonable coming from you)
Timing (must be done at the right moment)

If the joke doesn't suit these it will come off as creepy or stupid.

You can say lots of silly things.

"Hey, my eyes are up here."

This is the reversal joke.

"Are you comparing?"

Implying shes checking out yours but you are secretly slipping in that hers is nice as yours.

"Did I sit in something?"

This is to call attention to catching her looking because it can lead to more jokes.

"Careful its hypnotic."

Silly confident but it can open into more silliness.

"My other one is even nicer."

This is a play on a confident but at the same time deflection. Silly too.

Can you see the theme? I'm silly, borderline corny but I go for reaction to them rather than for substance. I don't care about sounding stupid.

Wow! this is really helpful. Thank you so much. I was thinking about the "my eyes are up here" one but was not sure if she would take that as me not being happy she is looking. I guess it is all in the tone you say it. Maybe I should give a little laugh as well?

Yeah delivery is important.

Thank you for your help. I may say some comment next time I catch her and the other women looking.

Grab some photography lover and post it here. Either way it'll be a case of pants on fire.