Is there something I can do to avoid a prolonged labor?

Exersise. Women in a good physical shape always has a better outcome with the pregnancies and labor, especially when they have good abdominal and pelvic muscles. It's might be too late now if you are in your last trimester, but swimming is still an option.
Not really. This is out of your hands really. Exercise regularly throughout the pregnancy. Don't overeat and really try to eat very healthy (lots of vegetables) so that you don't gain too much weight.