I'm fortunate in that this is my 16th year and I've been stiffed only once, and that was several years ago and for one mowing. It makes a difference that the bulk of what I do now are in one of several neighborhoods nearby.
Also, I just checked the mail today. I sent invoices out in Tuesday's mail. Slightly over half arrived in today's mailbox. I'm certainly not saying let people continuously take advantage of you, but it does pay to be friendly and flexible.
On a side note, three customers took the time to write thoughtful notes to me thanking me and telling me they're glad I'm back this year. As my wife likes to say..."that just gives me the warm fuzzies all over".