Spats, there is not a whole lot you can do. The more you can arch, as well as the lower you bring the bar (even though it sounds like you are bringing it pretty low), the less you will use your shoulders. Keep your elbows in as much as possible, for as long as possible. You will lose a little of the drive from the triceps by keeping them in as long, but your number one concern is not aggravating your shoulders.
What sort of problems do you have, and what are you doing about it?