First - ask for more details. Try to get them to email the details to you in written format. This gives you time to think and plan. And if they don't give you enough information, ask more questions.
Questions to ask:
- How large do you want the art?
- What is the surface?
- Do you want a reproduction or will I need to design custom art?
- How many people will need to approve the art?
- What dates do you want the art done?
- What is your budget?
Second - Decide on your hourly rate. What is your time worth? Do you have another job? What does it pay? Estimate the number of man hours it will take to complete, including your design time and all the time you use to do the business part (draft a quote, make a template, email a bill, etc.)
Third - Submit a written quote, either in an email format or fax. Spell out as much of the details as you can, so if there is a problem, it will be caught early, before you are committed. If you need to purchase tickets for travel, request at least half up front, so you don't get stuck with the cost.
Fourth - have the client sign and date the quote and return to you, as their approval of the costs. Keep all you emails and faxes until you are paid the full amount.
Fifth - The remaining amount should be paid once the art has been completed (rain or shine). Usually clients will give you a check at the event, or you can have money transferred electronically.
Do not undervalue your art or your time! You need to live and make a living just like everyone else. You may encounter some clients who think you will donate your time for free, but there are plenty of others out there that have a budget and will pay you. Who do you want to work for? If all of the artists charge fair prices for their time and art, we all win in the end.