What factors should I change for a dark roast in an Aeropress? I normally like a dark roast,  but I'm struggling to get as good a cup from my Aeropress as I do with lighter roasts.

What factors can I adjust to improve the flavors, compared to how you'd brew a lighter roast?  Less brew time? Colder water?
Dark roasts from an Aeropress easily become too bitter.

I would recommend keeping the water temperature just below boiling. Flip your Aeropress upside down so that you can control the brew time. Add your beans and water, and then brew for 30-45 seconds.

With lighter roasts, I do the above but let them brew much longer. I've found you have to adjust the brew time for darker roasts.