often times, the first problem for me is just remembering which way i'm supposed to be turning. remember that the threads are opposite on each side. if you're using a pedal wrench, you can put additional leverage on it by putting a pipe over the end of the handle. but, if you need that much leverage, there is probably some corrosion or metal seizure going on with these relatively delicate threads. you can buy penetrating oil at home depot that will soak in between the threads without damaging them. clean everything w/ degreaser and then apply grease before reinserting.

cheers,
jmb.