Once you have started optimising your charity's website, track how your search engine rankings change (see tip on evaluation for more details). Evaluate why your ranking has improved for certain keywords and apply this knowledge to increase the rankings of all keywords.
Track the number of visits made to your charity's website to see how this changes over time. You don't need to buy an expensive system to track this. I use the free Google Analytics tool to track traffic on my website.
So, what should you track? Although people often talk about the number of 'hits' their site is getting, this figure is an overestimation as every file that is downloaded (including image files) is counted as a separate hit. 'Page views' provide a more realistic assessment of the popularity of your charity's website. As your search rankings improve, you should notice an increase in the number of page views your site receives.
Don't just track the overall traffic to your charity's website, but also keep an eye on the most popular pages and the search keywords that people used to find your site. Evaluate why pages are popular and think about how you could replicate their success.
Also, make sure you take advantage of the success of your popular pages. For example, add links from popular pages to other pages within your site to encourage people to explore the rest of your charity's website. Or, try to convert those users into supporters or donors, by including a form to allow them to sign up to your newsletter, or a donation button.