You'll get to know which clients repeatedly turn up with mascara lodged in their lashes and it takes AGES to get out.
I advise them to wash their lashes thoroughly with Johnson's baby shampoo before they come for infills and, if they don't, I include the time it takes me to clean them as part of their infill time. I explain nicely that they are wasting their money by paying for my time to clean them when they could do this at home.
This usually does the trick.