How do you keep a pocket square from falling?

How small is it? For just about any regular size square, you can always choose a fold that will keep some of the square exposed outside the pocket opening.

I've yet to try it, but perhaps double sided tape.

Greetings, my first post.

There seems to be a few methods mentioned here or on AAAC

1) stuff something (tissue?) in the pocket first then the pocketsquare
2) tape? the pocketsquare to a small cardboard (seems to work better with linen squares)

fill the bottom of the pocket with another small square?

Try those 18"X18" squares! Seriously, does anyone use these? I have some silks that aren't too bad, but a linen sq. in this size is like jamming a napkin in your breast pocket, even when tightly folded Many stores carrying Irish linen sqs. seem to only have them in this size...

I stuff a small amount of tissue to keep mine from falling.

If this is a routine problem, you could tack through the pocket from behind, an inch or so up from the bottom with a few stitches of thread. This would effectively shorten the pocket. Removing it would be easy and painless and would leave no evidence.

Anti-gravity chamber?

I knew some girls in high school who used the same technique.

Starch it... oh wait, this isn't the Streewear forum.

The whole point about pocket hankerchiefs is that they should look as casual, not a series of rigidly geometric points. There are several classic alternatives, just look at any book on mens style. Or you can invent your own which is more fun. Having chosen a style, experiment a little with how you fold the hankerchief and after a couple of tries you will find something that works.

Our good friend Jon at FIHTies has white linen squares at about 11 x 11 for something like $25. Highly recommended.

Or you could get one of those "pre-folded" pocket squares and sew it in place.