To be honest, I usually offer to pay just to get the product early, I create a review site have it first page by launch day and then the results pay off for them self.

I usually introduce myself show them my sites to date for affiliate promotions. Havn't been turned down once. Also 99% of the time when they realise you are a serious affiliate they reply saying they would gladly give you the product before launch without charge. Otherwise I can not promote their product as you want the most visibility for a review site.