Find out if your prospective ISP offers a telephone number where you can reach a live person, and if they offer email support. Also ask how quickly they reply to emails. Ask if the ISP you are considering offers technical support or just customer service. There is a difference. Technical support professionals can assist you with the settings in your computer and with troubleshooting problems.
Customer service is more likely to just answer simple, general questions.
I have found that most ISPs who are confident about their service will offer a money back guarantee, or better yet, will not force you to sign a contract at all in order to get a reasonable rate. Again, look for what the prospective ISP tells you on their Web site. If they don't, however, be sure to ask before switching. Nothing is more troublesome to an Internet business owner than trying to get around the Web or access email using a slow connection. Find out what speed your modem is and then look for an ISP that can accommodate that speed.