How to make a good buy of SEO services?

[bob e]

Our site, http://www.newandimproved.com has fallen significantly in natural search over the last year. We have an active newsletter that gives great value for free (to those who find it) and we are all about 1 thing: Helping people and their organizations be better innovators. Clearly, the art and science of SEO work is growing more complex by the moment. Our philosophy is to try to find the best experts and hire them, rather than try to be experts in everything ourselves. We've played around with PPC on our own (and have low confidence that we're doing it wisely) but that's about it. The SEO market is so crowded with folks who hang a shingle (and declare themselves experts) that it is hard to even begin due dilligence to find a good, reputable provider. I seek guidance in both the criteria I might use to make a hire, and who's good out there. I'd also love to know how to know if someone is over-promising.

Thanks!