acetone or isopropyl alcohol will do it. do it outside cos of fumes.
if really built up pour solvent into a container and submerge the edge of a section of rim in the solvent for 30secs or so. remove sludge with wire brush / rag etc.
you only need to do this if the rim's really gunked up and the old glue is lumpy and uneven. if its not, leave the old glue on there as it will partially re dissolve when you apply new glue and help the tyre adhere better.
Before you loosen it off with a solvent and make it all gooey again try a buffing wheel with a real course brush. I have a bench grinder fitted with one at work for the soul purpose of removing old glue.