What types of customers have you worked with?
Local SEO services - getting clients' businesses listed on important local directories - Google+, Yelp, Bing, Yahoo, Superpages, etc - in a consistent and optimized fashion. Additionally, making sure that clients' sites are talked about in the geography that they serve - local business networking sites, local blogs, local business newspapers, etc.
WordPress Conversions - taking legacy websites built in html or a proprietary CMS and converting them to WordPress so that our clients have take control of their own site in a scalable platform.
Fixing Technical Errors - SEOs that do not have a coding or technical background often leave behind websites that are broken or wrought with errors that prevent Google from properly indexing the site. So essentially you could have a wonderfully SEO'd site in terms of keywords and backlinks, but if it's broken you don't have a chance of ranking high!
Keyword Research & Writing - Let's discover what your keywords are and then get them in the right places on your site.
What advice would you give a customer looking to hire a provider in your area of work?
Make sure that you ask for examples of previous work, case studies, testimonials and references. There are a lot of SEOs selling snake oil - basically buying an SEO software and up-charging the customer - thus adding little or no value.