White Hat SEO
You do not need a hat to hire a SEO but the SEO expert you hire should have one, a white hat. He should wear it while working for you.
White hat SEO is clean user oriented search engine optimization that neither alienates potential clients nor leads to penalties by search engines like Google. You do not need to trick or game search engines to rank high. White hat SEO offers many methods to optimize web sites, gain link popularity and thus high rankings, most notably link baiting.