1. Effeto Mariposa remover does work faster and doesn't evaporate as fast as the alternatives. The best out there, though nothing makes it easy.
2. Heat. Put the rims out in the sun for a couple hours if that's enough to make them warm. Put them next to a space heater. Use a hair dryer on low setting. Obviously, don't do this if you've put any flammable solvent on the rim already.
3. Scrapers. Get a scorp from www.woodcraft.com
or other woodworkers' supply. Or use a grinding wheel or file to reshape a chisel or scraper that's about the width of the tire bed. You can also simply cut foot-long lengths of galvanized electrical conduit, scrape off any burs on the outside, and put a crutch tip over one end. Then the other end makes a very nice scraper. You just cut off a couple inches when it gets all gunked up and repeat. This conduit is cheap and throwaway. This is usually my favorite simply because it works as well as anything else and I don't have to clean it up afterwards.
But generally, cleaning the rim off shouldn't be a frequent action. You build a better, stronger glue base after a couple tires have been on the rim. Excessive water, dirt, or mixed glues are the things to avoid, so cross wheels do need more care, but also you can use additional techniques there such as sidewall sealants to prolong the glue life.