Have you ever had a client come to you with vague goals? When they simply state that they want their site to rank better or that they want more contact us form submissions, it's important for you to determine a baseline starting point before you begin any optimization. If you don't have any measure for where they're starting from, it will be exceedingly difficult for you to prove how awesome you are after you complete your optimization efforts.
The measurement methods described below will allow you to communicate with your client exactly how much your work has improved their site.
This list does not include third-party measurements such as Pagerank, Alexa or Siterank. These external measures are too easy to fake, providing false results. Frankly, having a high Pagerank comes naturally after doing a lot of hard work in other areas -- it's not a measurement that in and of itself should be used to determine how successful your site is.
Increasing Conversion Percentage
Every site that you work with needs to establish a primary goal. Perhaps it's making a shopping cart purchase, perhaps it's having a user complete a contact us form. The conversion percentage of that action is a key measurement for you to pay close attention to.
Google Analytics is the easiest tool to use for tracking conversions, although for small projects Website Optimizer is a good utility as well. For complicated multi-step processes like shopping carts, Analytics is the preferred measurement tool over Website Optimizer.
By tracking the rankings of keywords as they rise and fall over time you can measure your link building efforts. If success for your client means that you need to be on the first page of Google for a particular term, it's important to know if the site ever enters or exits that realm of success. Here's a full list of utilities to help you track keyword rankings.
Optimization does not have to be limited to on-site efforts. Improving clickthrough percentage from the SERPs is a worthwhile task. By working to improve individual page titles (and even seemingly minor details like meta description), you can indeed work to improve the clickthrough percentage of visitors searching for your target terms.
Tracking Phone Calls
The ability to track conversions or leads from phone calls is not the nebulous task that it used to be. The emergence of Google Voice means that creating a unique phone number for any particular advertising campaign is a simple task. Creating too many numbers can quickly become overwhelming though, so you'll need to segment your campaigns wisely. Head here for more information on tracking phone calls.
I hope that these tracking techniques can help you to quantify your optimization efforts in the future.