farecast.com provides a "fare forecast" tool for many combinations of U.S. cities.

Actually, I'm seeing a lot of $352 fares for DEN-DTW in August. But maybe you need specific days.

I haven't had much luck refaring. I believe that refaring might not help you if (a) you purchased the original ticket using some kind of voucher or coupon or (b) the lower fares are in a different fare class than what you originally purchased.