Ye Olde farmer fix...If you have a good quality tablesaw, tape a piece of emory paper or the appropriate grit sandpaper to the table (tape all 4 corners down, dont roll the tape over on itself). The good quality tables used to be checked for flatness...so this will help you remove the requisite material, and you can use a feeler gauge to see where you need to remove material. Also, make sure you move the object in more than 1 direction...either in circles or an X-pattern.
No tablesaw table? use a piece of window glass