If you provide services that may result in injury, illness (eg. food services) or death (skydiving or other extreme sports), loss of property (eg. theft from a locker), or permanent changes to physical appearance (eg. tattoos, piercings, surgical cosmetic enhancements), you should have every customer sign a Waiver and Release Form BEFORE you provide the services.
This rule even applies to home owners who allow minors to use a pool, hot tub or spa on their property. If you can envision a situation where someone might sue you for damages, you should be using a waiver and release.
In certain circumstances, you will also want to have the customer indemnify you against any costs or damages you incur as a result of your dealings with them. For instance, a catering company would want the client to indemnify them against any legal actions brought against the company by guests attending the function who became ill due to reasons not caused by the caterer's negligence.