Lou, that is what we all want to be and it is not something that you will learn from books or conferences or chatting with other shoppers. It is something that you accomplish by reading instructions carefully and following them, shopping the correct location in the correct time period, collecting whatever proof of visit is required and submitting it and your report in a timely fashion. Be objective and precise, "There were three employees visible in the store." rather than, "There wasn't much help available." "The walls were purple." rather than "The walls were an ugly grape soda color."
Don't have excuses. There is not much you can do if someone totals your vehicle while it is parked in a parking lot, but be communicative and creative with your scheduler when you request a reschedule of the shop rather than a straight cancellation (or worse still just ignore the shop and not do it at all). Suggest you need to reschedule and will be getting a rental vehicle until the insurance sorts out the claim so will be able to shop. Try as much as possible to perform shop at the beginning of the 'window' (i.e. you can do the shop M-W 8AM-6PM, try to do it Monday morning) so if you walk out the door and find you have 4 flat tires you can get it sorted out and do the shop on Tuesday or Wednesday without needing to contact your scheduler at all. Similarly, you get home and discover you shopped the wrong location (which happens to all of us some time) you will have Tuesday and Wednesday to get the shop done. If you leave shops to the last possible time to perform them and a mishap occurs, you will need to communicate, apologize and try to reschedule. Sometimes your scheduling and sanity require you do the shop at the last possible moment, but recognize you are taking a risk.
Keep good records so you won't overlook that you had a shop to do today that you missed doing because you failed to write it into your schedule. If/when that happens (and I think it has happened to all of us at some time or other) call as soon as you discover the error, apologize for your error and ask to reschedule the shop ASAP.
Look at your receipts. Sometimes a cash register is sent over to location B from location A. You went to location B but the receipt still shows the address of location A. You visited the location on October 22 and for some reason the receipt reads June 22. You visited the location at 2PM and the receipt says 10AM. Before you leave the location you want to make sure you got YOUR receipt so that it shows the required purchases. You really can't argue with the location about their address, date or time stamp without being obvious. But you should proactively alert your scheduler that there is a receipt issue and why you are sure you were in the right place, on the right day at the right time. If there is a space on the report you are entering to make a similar notation, do so (a comments section or similar).
You really do learn this business by doing because despite their similarities, every company is a little different and every client report is a little different. Diligence, good will and reliability are your best allies.