I have about 10% to 15% outstanding at any one time. So if it's about that, I wouldn't worry. So long as they pay by the next clean. To get your set up this far, you need more customers than you have at the moment. If you had enough customers, you wouldn't have a cash flow "problem". So the best way to solve any problems with this game is put more customers in your business.
If you can put one customer in, you can put another one in and so on and so on. Keep doing this untill you can't keep on top of your work, then get rid of the customers that pay too late or not at all. Then carry on putting customers into your business.
Notice a pattern here?