Once the basics are covered I've found having fresh (but pertinent) content makes the biggest difference if you or your client are not paying for an ongoing SEM campaign.
If you decide to stick with it you'll notice (if you haven't already) that there is a lot of free WP info out there (not all of it good, unfortunately). There are also some resources that are minimal cost that can really help out, for example - http://digwp.com/. This puts a lot of useful info...