SEOmoz has a great round-up of tips for doing SEO on an existing web site.

Let Google Analytics run for two weeks before doing any SEO - This allows analytics to collect data and provides you with a baseline. I recommend you screen capture the relevant pages so you can show your client how your work has positively affected their site.

Evaluate the visual design of your client's site - If the site drives people away, no amount of SEO efforts will help. If the site looks terrible, find well designed sites in your client's niche.

Check compatibility between browsers - Visit your clients site using Mozilla Firefox 2.x and Internet Explorer 6 and 7 and Safari 3.x. Remember to do this on both a Mac and a PC. You want to make sure your client's website renders correctly and won't drive away users.