There is really no easy way out... I have completed at least the local pokedex in every pokemon game iv'e gotten, and i just did it by stocking up on pokeballs like crazy, going online and printing off a checklist, and sometimes by going onto some kind of pokemon wiki like serebii.net to figure out where different kinds of pokemon appear. for pokemon you really just cant manage to find, the GTS is a good place to turn, as long as trolling noobs arent there to ask for impossible trades...
anyway, good luck!! hope this helped
Do hard things. Never leave your man card at the door.