It's mandatory to track your key performance indicators (KPIs) and measure return on investment (ROI). Your KPIs will be based on the most important metrics for your site category.

• Ecommerce sites measure sales, returns, shopping cart abandonment rate, sales per visitor, cost per visitor and conversion rates.
• Content sites measure visit length, page views, number of subscribers and cancellations.
• Lead-generation sites measure white-paper downloads, time spent on the site, newsletter subscribers, abandon rate on contact pages and leads-to-close ratios.
• Self-service sites measure the decrease in visit length, inbound call center metrics and customer satisfaction measures.
• Social-networking sites measure page views, the number of members, number of profiles, changes to profiles, number of member interactions, et cetera.

Once you monitor these metrics over time, take appropriate action to improve your site's effectiveness by optimizing for greater ROI.