I know it sounds kinda crazy but I like to be able to show my customers I did them a service in some concrete way. If I know a system isn't cooling well I'll take the vent temps first and show my customers what they are. After the service I take them again and show the improvement. It has really worked well for me so far. I just have a quick look around to make sure that I'm not servicing a system there is nothing much wrong with before I put on my before and after show. Anyone else tried this?