I have thought about this a lot in the past and came up with a simple solution.
The easiest way would be to track the search terms and the ip address/mac address used with the search terms. If google sees them come back to the site and google the same/close search terms as the last link the last link they had is -1. If they are seen with different unrelated search terms the last link they clicked for those search terms is +1. If they arn't seen back on the site in x minutes, the last link they clicked gets +1.
The easiest way would be to track the search terms and the ip address/mac address used with the search terms. If google sees them come back to the site and google the same/close search terms as the last link the last link they had is -1. If they are seen with different unrelated search terms the last link they clicked for those search terms is +1. If they arn't seen back on the site in x minutes, the last link they clicked gets +1.