I did the old trick of using large amounts of Liquid Wrench, putting a box-end wrench on there and then taking a hammer to the wrench. The shock helps the Liquid Wrench work, and helps break loose the rust.
By the by, I also did the bottom one first. I figured it was gong to be more difficult, and that it would be useful to have the top bolt still in there to keep things anchored and stable. It may have had no effect, but it seemed logical to me.
