If you hire a "SEO expert", you should ensure that they use legitimate, "white hat" optimization techniques. If they are continually trying to trick the search engine algorithms by using black hat techniques, it can get your site banned from the search engines.
It can also get very expensive to hire someone to do SEO for you!
So we prefer to do the SEO process ourselves - it`s not difficult because the website hosting and marketing system we use has built-in tools for keyword brainstorming, search engine optimization and submissions, to guide us through the process.
I hope this helps.