I use stickers from the Brother P-touch series. They are easy to peel off and don't leave a residue. I have found stickers to be so helpful for keeping track of my own racquets and for when I run into customers on the court. I can look at the date and quickly tell them how long it has been since they had a string job. They seem to forget that my stickers are there at all and are surprised how long it has been since their last stringing.
That is good directional information but, alternatively, you can just carry an ERT in your bag and check the customer's DT, giving them a better idea of what's actually going on with their racquet. The P-Touch labels are much cleaner in both appearance and removal if going the label route.