Good question Sina,
In my experience this is one of the more difficult areas to track, especially long term. Tracking should be built off of the deal thesis, but allowing for other areas of opportunity to be identified and added. A good structure should include what to track and how to measure, which will obviously be different for every deal and enterprise depending on the level of data they are able to obtain. A good CRM system is extremely useful, and any campaigns that are developed to cross-sell, etc. can be added as a field for tracking purposes. Ideally the revenue synergies are identified as separate targets for sale leadership, so there is no incentive for the wins to be miscategorized – give them two targets that must each be tracked and reported on as part of their overall accountability process.