Gentlemen,
I respectfully recommend that you obtain a purchase order number (PO) whenever possible before proceeding with your work. If you cannot obtain such legal qualification state in your proposal that payment is due within 30 days, 15 days, upon receipt of the service (i.e. drawings, report etc), or some other period that is acceptable to you. Put in writing that which you propose to do, when you will complete your work, what is to be delivered to your clients and the terms for your compensation (amount and time for payment).
If you are working with an established business such as a manufacturing company they will comply (they also expect you to deliver as written).
However, if you are unsure of the character of the client then ask for a retainer of some substantial amount that will allow you to finance your beginnings of the project. If nothing else their hedging on giving you a reasonable retainer should tell you something of their intentions. Also, for those clients that you have doubts about, Do Not deliver the goods until you have payment in hand.
Many of our clients, the people that we deal directly with, want to honor our agreements (usually payment is net in 30 days) however, their accounting departments often find creative excuses for not meeting that deadline. With the economy the way it is (mediocre) many of our clients deliver payment 45-60 days after receipt of our invoices!
As stated previously by cb4, this is a business - and you have entered into an agreement either verbally or written, both of which are enforceable in court. Don't let unscrupuluos clients take advantage of your honest efforts.
If I may ramble.... one contractor stiffed me and my company for a measly $600 dollars. I took him at his word and he didn't pay. So I filed a mechanics lein against his "company". (He worked out of his house doing small time renovations). The mechanics lein made it impossible for him to acquire any additional credit or financing until the lein was paid in full. If he had paid after the first month we would have split the difference with the collecting company. After the second month we would have recieved 25%. This contractor did not pay for a full year. But when he needed financing, he did come up with the money and he paid the collection agency the $600. That was fine with me. We didn't get what was due to us, but he didn't get to keep what wasn't rightfully his. Stick to your rights, be consistent and fair with the clients that are willing to work with you; and for those that lack integrity let them find someone else to take advantage of, after you get your payment.