Not getting paid in a timely manner is probably the #1 reason for cash flow problems. The longer someone owes you money, the harder it is to collect. So make it easy! Investigate ways in which you can get paid quicker and frame it as a benefit to your customers:
Create a service package and have they prepay. The benefit is they don’t have to write so many checks, and maybe you create a discount incentive to go along with it.
Also consider accepting credit card payments. The benefit is they get reward points on their card, and you get paid. Set up for automatic payments and you get paid even faster!