well... when you get your product, check it out. use it. yhen. once you're done, write about it. honesty is always good. even though you received the product (at no charge), if there's something you didn't think was great about it, be honest. make sure it's legit, though... not something like... i didn't like the shade of orange.
then. hold your giveaway. you can have people do a number of things from checking out the sponsor's site and commenting on their favorite item, to tweeting about your giveaway, etc.