Metrics: How to measure the effect of optimization?

Improving content and navigation should be based on some measurable characteristics (metrics) which allow to asses effect of the changes on the final result. For example, if the site's primary function is generating business leads, the main metrics would be the number of site visitors who initiated the contact. Analyzing user navigation patterns helps to clarify the reasons why some of them leave the site quickly. It's clear that only a fraction of the contact forms filled out on the site will result in sales, that's why it's difficult to assess the payback based on efforts spent on the site development. However, in most cases these efforts are worth it. For instance, deploying a system for registration and classification of the filled out contact forms and its integration with a corporate CRM system allowed to make conclusion that the site brings contacts requiring a relatively low-cost presales investment.