Look at the example email below for the full list of questions.
NOTE: This email has been enhanced to now ask technical questions about Analog, ISDN, xDSL, and cablemodems.
After you start getting replies from the ISPs, create a text file and start ranking them on price, speed of responce (gives an idea of their support), features, etc. This can take a long time since many of the responces you will get will be automated responces without any relivant info. Then you have to re-transmit the e-mail to a different address. But after all this.. you will have a good idea of who will serve you the best! Here is my ranked list of Walnut Creek ISPs that I compiled up. This took me over 2 weeks and hundreds of e-mails!
Finally, try to get some reviews of your top  ISPs. Check out my [DSL] and [ Cablemodems ] pages for some evalaution URLs to look at.