we have had great success calling a restaurant from our resort in the morning before we head out to the parks... there are certain meals that won't be do-able (i.e. crt) but most restaurants should be able to accommodate you, although you may have to be flexible with time and meal (i.e. you may want cp for breakfast but are offered a lunch ps)...
you can also call from within a park during the day to see what availability is... so if at 2pm you are thinking you will end up near a certain restaurant in a few hours, go to a gift shop, ask a cm for a phone (they will either let you use theirs or direct you to one) and call the restaurant you are thinking you'd like...
another rule of thumb is that tables are easier to come by in the resorts than in the park... and rather than wait 2 hours for a table at a park restaurant, you could travel to/from a resort restaurant in less time and have an enjoyable meal (from mk i'd suggest whispering canyon at WL or 'ohana at the polynesian; from epcot i'd suggest any of the restaurants at the swan/dolphin; etc...) just be sure to CALL FIRST!
good luck and enjoy!