Marinating overnight with a few good squirts of WD40 prior to pedal removal makes things a bit less difficult. Unless they're right royally stuck. Then you have two options. One is a length of pipe over the handle of the spanner to give you extra leverage, and two, if that fails to budge them, your LBS should have the tools and nous to loosen stuck pedals.
+1 for applying a generous slathering of grease on the spindle threads prior to reassembly.