Be human. That's all it takes.
Many of the companies I hear from have a "press release" type of "blog post" already written up that they simply want me to post, verbatim. More times than not, these companies and their products have nothing to do with my field (networking) and are not the types of things that I would write about or that "my readers" would expect to see on my web site.
That doesn't stop them, though. It's like they came across my web site and thought, "hey, here's a blog, let's e-mail them" without regard to "my audience".
Primarily as a result of some events I've been invited to/a part of, I've had the opportunity to visit and get to know many people in the networking industry in the Bay area (I'm just a networking guy from the midwest). While visiting and meeting up w/ folks from Cisco and Juniper is cool, I'd much rather read and write stuff about start-ups.
Basically, if you want me to write about your product, get it in my hands. Whether it's a web app, mobile app, or physical product, I, amazingly enough, want to actually use it for a while and form my own opinion. If you expect me to simply regurgitate the great things you've said in your "press release", you're mistaken.
What could be the right incentive? Exclusive data (for bigger blogs), early access, promo codes, discount codes, free subscription, valuable content as a guest post, the list goes on.
Just asking for review regardless how cool is your project is not enough.