Before you write that big check to some SEO poobah who's just sold you a bunch of SEO snake oil consider the following salient point: SEO is NOT a science!
With legitimate hard sciences you can measure, compare and contrast and draw conclusions. Not so with SEO. The numbers change by the minute. So do the yardsticks. When search engines first hit the scene in 1994, keyword HTML tags and keyword desnity were the top determinants of page rank.
Then, all the yokels got on board, stuffed keywords everywhere, thus diminishing the value of the SERPs. So a new set of criteria are developed - one that won't be so easily circumvented. Inbound links. The more inbounds from quality sites (sites that rank higher than yours) the higher your site ranks.
Problem is, there are very few rules in this game of "Who's Tops?" There are no axioms (a=a). There are now laws, though there are plenty of ways to shoot yourself in the foot by using black hat or gray hat tactics.
Want to learn more about what the experts feel counts for and against websites. SEOmoz.org regularly publishes it's ranking factors based on input from SEO experts.
Consistantly, there is no single rating factor that all practitioners of the SEO arts agree. NOT ONE.
So, all SEO pros are guessing at what search engines are looking for, just like you.
And because SEO lacks empirical data, you can be sure that you'll be hearing excuses from that high-priced SEO about why you're site is still buried in the backwash of SERPs.