Overbaking is one of the most common ways to ruin a pan of brownies. Keeping track of the time is the best preventative measure, but using a clear glass pan can also help.
Set a timer for ten minutes less than you'd normally let the brownies bake, and monitor the oven for signs that the brownies are done, like crispy edges forming.
When the edges of your brownies slightly pull away from the sides of your pan, they're done. Make sure to take them out as soon as possible, or else they'll continue to cook.