Work on increasing perceived professionalism: no lawyer in the history of the legal profession ever had this problem. (Slight exaggeration, but not much of one.) This is a broad topic, but in general, be more communicative, comport your affairs like you're a serious business (contracts, professional language, business mien) rather than a kid cutting lawns for beer money, etc.

As a stopgap, offer to invoice after the month, not before: "That way if I'm not there when you need me, you don't have to pay."