In my opinion, it really depends on setting the expectation of the client.  Some clients want to see ranking increased, while others are satisfied with maintaining the same ranking yet converting more of the existing traffic.  Most of the time, my experience has been to do quarterly agreements with my clients and spell out the t/c’s of what to expect from a measurement/metrics.  Obviously, if I don’t meet those metrics, then there is a good chance they are not going to renew the quarterly agreement or possibly extend to an annual agreement.  Again, set the expectation for what the client will receive and tie it to their revenue, then you’ll be more apt to gain the trust and further business of the client.  For new clients, I also give them a 15-30 day trial run depending upon the client relationship and where I think I can take their site.