How do you actually get paid the money?
RobGR: I am not the best person to give advice on this, because I have had only a few customers that were big companies.However, the standard for payment is still a check via US Mail.  If you anticipate slow payment, bill early and include some standard text on the invoice about net terms of 30 or 90 days, whatever is appropriate, and if you wish specify that late payment is subject to a 2% per month late fee.  This is fairly common, although there is a wide variety of practices.If they wish to pay with a credit card, I send an invoice using the Google Checkout email option.  This costs me 2% of the bill, but it is probably worth it to get paid early, and I eat that 2% (but treat it as a business expense).Some companies and some industries are known for ill treatment of contractors and vendors, such as habitual late payment, suggesting that they will only pay the outstanding bill if you agree to some future work, etc.  I suggest you simply cut off all business with anyone who starts treating you like that, one of the advantages of being a startup is that you don't have to put up with shit from "long term customers", so don't get fresh new shitty customers.