Since you don't have highways i hear, you could always just drive it aggressively, rev it high and rev it hard. This will of course use more gas, but it will help clean up the system. May i also suggest getting a can of carb cleaner, and cleaning up the throttle plate and surrouding area on the carb. Also, you can do this with the car running and the airfilter and stuff taken off, and shoot the carb cleaner into the carb's throat to help clean out the system a little after the carb throat and throttle plate that you will have already cleaned. Then just use some good gas and go for a nice hard drive...

Also, there is nothing wrong as far as i'm concerned with the method you(Erling) originally suggested, of taking out the plugs and letting it sit over night with something inside. Another way to do this, is to have the engine running and have a slight stream of water going into the throat and this will also help with cleaning the engine, and can be done instead of the letting it sit overnight option...

Good luck!
--
Kyle - attending Ore. State, while my lil 68 142 (74 b20, broken m40) awaits... (check out my cardomain site, kneedsacar)