The single most important factor is the local dial-up access number. You don't want to incur long distance phone charges when connecting to your access provider. Outside of high density population areas, local ISPs are your best bet. If there is no local dial-up access number in your area, further research is needed. Some ISPs offer toll free 800 dial-up service at rates lower than long distance phone calls.
Some companies also offer lower monthly rates for limited hours.
Some ISPs offer a trial period or first month for free. Billing is most commonly done automatically by credit card, but some ISPs offer monthly billing by mail, or provide the option to pay for a year in advance. In addition to these issues, consideration must also be given to customer service and reliability. Ask others in your community what they are using and if they are satisfied.