CWLee wrote:

Background: My home office area is carpeted, so many years
ago I purchased, from Staples or Office Depot, a plastic
floor mat, sold for this purpose, about 4' x 6', smooth on
one side and with an array of sharp points on the other
side. My desk-chair rolls on this very nicely, and the
sharp points keep the mat where I want it on the carpet.
Gradually depressions have developed in the mat, probably
because the wheels of my desk-chair tend to rest mostly in
the same place.

I'm looking for a way to get rid of these depressions, and
for the mat to be flat again. My first (and so far, only)
idea is to place it, smooth side down, on my concrete patio
in the sun on a hot day - thinking that the heat might cause
the mat to return to its original shape.

Any other ideas or experience out there dealing with this
problem?

That is probably the best and easiest thing to try. Those mats come in
different grades and the thicker more expensive ones resist denting
better. Also note that home carpets typically are much deeper pile than
the commercial carpets these mats are intended to be used on so the mat
is already over stressed. If the dents don't come out just buy another
one.