From experience, I could never get mine flat again. It only curves a bit on one edge.

I don't think it takes much to throw your cutting off by a hair... it might be time for a new one.

This is one of about 20 reasons why I purchase Martello cutting mats ONLY ( I wait for the 50% off Redline Direct sales),,,The mats stay in shape, never warp, are kind to my blades, etc etc - and probably most importantly, every one of their lines and markings is 100% accurate! I can use the mat to measure and be on the mark 100% of the time.......

I have flattened my board by laying it on the hood of my car when the motor is hot and weighting it down. It worked for me.

I did the glass fix with a warped mat. The mat was quite old, but it worked. It turned out not completely flat, but much better and doable. To find the directions for the glass fix, you need to scroll way down to the comments. It is called "Charlie's fix" here.
Eventually, I did cut that mat into two pieces. Gave one half to my daughter and use the other half when I sew in the living room. When I want to cut something bigger, I have to go into the sewing room.

Thank you all for the quick responses. I have a hard time throwing such expensive things away, it is a very large board. So, I will try the suggestions, but will cut it up if they do not work. You all rock! This is the quickest response to an online question ever. Usually, even on blogs from my other activities, it takes forever.

Buy a lazy susan and cut the mat to fit the top. You will use your rotating cutting mat a LOT!

I'll try that too! Thanks yngldy!

Unfortunately when the mat gets wavy the measurements on it get distorted enough to affect your quilt results. Best to buy a new one. But, if there are any flat areas you can cut these areas into smaller mats.