The giveaways change every day and you can even request for a unique Pokemon. For easy updates/reminders on your LJ friends page, just add makeawish_gts!
All the instructions are here. Once you REGISTER for an account, you can even customize the giveaway Pokemon (nickname, moveset, held item, and more). You can even transfer your favorite Gen IV Pokemon over to Black/White without the use of 2 DS systems!
(If you have any questions that aren't answered in the FAQs, the Make a Wish GTS creator is great at responding to members!)