You need to have a clear policy that you are willing to enforce.
A customer that doesn't pay is not a good customer.
We agreed to a price, I'm willing to bill it because not everyone writes checks everyday (heck,I've never even met some of my customers, I just send them a bill and a check shows up) but I do expect to be paid and paid on time.
If you can't pay on time, it should cost you money. I'm being a bank, banks charge interest, so do I.
After 3 weeks, you get a friendly reminder call (people do lose the bills, they get lost in the mail, whatever, stuff happens)
After 1 month, you get a "you're late, you now owe X + X%"
After 6 weeks, you get a not so friendly reminder call.
After 2 months you get another letter with X+2X% now due and the bill is not satisfied unless the late fee is paid and in 2 weeks they will hear from my lawyer.
After 75 days, you get a call from my lawyer and now you owe lawyer's fees on top of it
At 90 days, the next step is to lien the property (which is yet more lawyer's fees).
You have to be hard nosed about this. The squeaky wheel gets paid, the silent one gets ignored. You wouldn't work for free would you? Of course not, so why are you?
it's by far the worst part of the job. I've never gotten to the lawyer part, but it's all set up to happen. It takes a lot of work and frustration to get paid from some people, the interest pays for that time that I had to spend to call/write/cajole you to pay your bills.
Oh, and people who pay late all the time, they don't get the best pricing. it's only fair. And anyone over 30 days doesn't get anymore work done (obviously).