I do consulting and I get a written agreement giving rates, total amount, and other terms. Usually the company issues a purchase order and I invoice against the purchase order, usually net 30 days, 15 if I can get away with it. They send me the checks. I dont give discounts. I havent worked with mom and pops. My clients are multi-million to multi-billion corporations and I havent had any collections problems. It's may be more problematical with sole proprietorships and such. My billings are typically 5 digits per month. If you are talking a few hundred per year, the purchasers usually expense it on a credit card because the PO process is so cumbersome.

Getting large companies to pay on time hasn't been an issue, but you need to agree to their idea of a schedule. Sometimes 15 days works, but one insisted on 45 days and the 45 days was when they put the check in the mail.

See http://news.ycombinator.com/item?id=438740 for another perspective.