Best way to remove excess solder from sweat fittings for re-use?
What you're doing is what I've found to be most effective also.  Don't add any flux to the fittings, however, add some to the steel wool since you want the solder to cling to it and not the pipe fittings.
Keep in mind that you're not going to want to reuse these for any sort of drinking water use since they were originally soldered with lead-based solder and it's nearly impossible to get it all off.  But for your heating system use or other non-potable water use, no problem as long as they are in good shape.