In my humble opinion, the best way to track uplift from cross-sell would be to segment your whole site traffic on those that bought and did not click vs those that bought AND clicked on a cross-sell link (obviously all cross-sell links would need to have a unique identifier). Then you could see exactly what uplift on conversion, items per order and average order value you get on standard buying traffic.

Your cross sell/upsell posts are some of the best anywhere – truly. I put together a post on cross selling and upselling as well (I linked back to your “Cross Selling Tips for Online Retailers”.)

I’d love to get your thoughts (if you have a minute) on this post here:
http://sellitontheweb.com/blog/tips-effective-cross-selling-up-selling/

@Tim – thanks for the tips on how to measure this in Google Analytics.

addtobasket/fromproduct/’productadded’
addtobasket/fromxsell/’productadded’/'fromproductpage’

Doing something like this allows the GA “Content Drilldown” report to work really well. You can start with rolled-up figures for all the adds to baskets, then drill down to get greater detail.

You would want to have a long think about which way round to structure the last part of the cross-sell path

addtobasket/fromxsell/’productadded’/'fromproductpage’
or
addtobasket/fromxsell/’fromproductpage’/'productadded’

depending on which version of the information would be more useful for making changes to your merchandising efforts. I guess you could even stuff all the different xsells offered into a ‘productadded_not-chosen1_not-chosen2′ monster string and then parse it out later if you wanted.