A few things worth considering:
(1) What level of risk are you willing to take with your site? Are there any methods that you don't want used (e.g. backlinks from specified sources, hidden text, doorway pages, ghost pages, cloaking)? Are you buying white-, grey-, or black-hat SEO?
(2) What level of control are you willing to give to the SEO? Will they be allowed to re-write page titles, internal anchor text, and even the main content on your site? Or are they expected to tweak things behind the scenes as far as possible?
(3) What counts as success? Are you trying to achieve high PR, or (more likely) top-x position in the SERPs for specified keywords? If the latter, what is the target position and for what keywords? Under what circumstances will the SEO be deemed to have failed?
(4) What's your time-scale for achieving this? (This will need to be negotiated with the SEO.)
(5) Are you hiring for a one-off optimisation, or are you willing to pay an ongoing fee for monthly monitoring and maintenance?
If you're show examples of successful optimisation, then look for sites that have been established at the top of the SERPs over a period of time, not just for one day in 2002.