Web.Config : Configuration by Environment

During a web project lifecycle, we encounter always the same problem when deploiement is required in development environment(Server A + DB A), Receipt/Qualif (Server B + DB B) and production (Server C + DB C without taking care of web farming). Many developpers choose to create one web.config for each environment; not a bad choice but a permanent synchro is required when adding reference or others commons configurations.
I was looking to improve this system to minimize the synchro time between each web.config file and find a simple tips to exclude the specific data from a web.config : the configSource property.