First, don't let it get old.
Second, soak it overnight in salt brine to plump it and make it juicy.
Next, cube it and sear it, then pop it into the pressure cooker. No pressure cooker? Get one. They're awesome, and keep your house cooler while making soups, stews, etc faster. (Reason is that you can raise the temperature above 212 degrees farenheit.)
Brunswick stew is better with more stewed squirrel.