We meticulously monitor all of our SEO projects for changes in their Google Analytics for clues as to what is working and what requires more attention. We often compare monthly changes by year, and at the end of the year, we like to see our progress and determine how much value we provide.
Here is a good example of one particular client's data, comparing 2012 to 2013. This is a real client and this is real data. We took over the account in April and by the end of May we had redesigned their site and were in the beginning stages of their SEO program.
Over the first couple of months, we saw a bit of a dip in their click-through rates, so we let Google's Web Master Tools guide us to correcting many issues. In particular, we used 301 redirects for the content pages that were either moved or jettisoned. There was a problem with duplicate content that we cleaned up and we implemented a more detailed tracking methodology to fine tune our efforts.
In the end, we saw a significant bump in overall traffic, and the most important metric, more conversions.