Money rules for freelancers (from A List Apart)

1. Set your rate and the associated deliverables up front. Get a signature that binds both you and the client to the same ground rules.
2. Never discuss your rate with a client’s staff member. It’s none of their business what you’re paid and it’s usually more than they’re getting per hour (risk, insurance, and cost discussions set aside for now).
3. Show up and do the job on time. Your work is usually subjective and a responsible reputation can help you influence the outcome of a design presentation, an application demo, a product review or a crucial conversation.
4. Don’t treat your invoices with an apologetic tone. You and the client agreed to a rate and a schedule of terms. Submit the invoice and thank them for their prompt payment.
5. Fulfill your obligation. Even if you have to bring in a resource under cover of darkness and eat the cost of the extra help, do the job you agreed to do. This will not only get you prompt payment, but will also boost your reputation.

More here.