How do you keep the collar on a polo shirt from curling under at the points?

I iron it, but somehow throughout the day it tends to curl under?!?! so frustrating! and sloppy

Spray starch is a bit wimpy for this job so you can make your own starch instead.

Go to the grocery store's laundry aisle and pick up a box of ARGO starch. A "medium" starch should work well without your collar feeling like cardboard. Follow the directions on the package, but only dip the collar of your shirt in the solution.

Wring out the excess starch and lay your shirt face down on a towel to dry (overnight should do it) and make sure you smooth out the collar points.

The next time you wear your shirt, the points will be nice and straight and they won't curl up on you!

Remove it from the dryer while it is still damp and use a clothes pin on the points of the collar to hold them straight. I also do this on pant pockets that curl up.

Figure out exactly where and how you want the collar to lie, then pin it down. Turn the shirt inside out and tack it down with a couple of stitches.

For men's shirts, there are inserts that can be put on the points that keep them from curling. I don't know if there are similar things for women. If all else fails, extra-stiff starch.

This answer will date me, but here goes. Shirts used to come with little plastic inserts sewn into the collar to hold them down. I have added those to shirt collars myself in the past to help keep collars laying straight against the shirt. I used to make the strips out of plastic gallon milk jugs. It sounds wierd, but does work. Make a tiny slit under the shirt collar and insert the sliver of plastic at an angle and use a needle and thread to secure the sliver in position, from underneath, making sure the needle does not penetrate through to the top side of the shirt material, so it is invisible. Use a little spray starch to lightly iron the collar and remove any wrinkles and you are through.
The strip of plastic is usually about 1/4 inch wide x 1 3/4 long.
I think some men's dress shirts still come with these.