After a few washings, my cotton polo shirts seem to develop hard
creases that run the full length of the collars, and they keep the
collars from laying flat. These creases are so stubborn that often
ironing won't get rid of them. I've gotten so frustrated that I've
actually thrown some pretty
expensive shirts away because I can't get rid of these creases or get
the collars to lay flat.

Is there something special I can do when washing or drying my shirts
to prevent this in newer shirts, and is there anything I can do to get
these creases out of  collars where they've already set in?

I have the same problem (I'm hoping someone else posts a better
solution than what I've come up with).

If you unfold the collars and have them stick up (like folks in the
'80s used to wear them) immediately after washing, it seems to help a
bit.

I've resorted to ironing some of my shirts to fix this problem.