Here's a rough guide to the best schemes for various types of users. But be careful. Check the fine print, and when in doubt go for an ISP with a number of pricing policies which will allow you to change if your needs change.

Simple business use (send and receive e-mail, and perhaps a small amount of Web browsing). Set per-hour fee. Make sure it isn't levied with one hour as the minimum chargeable unit. And check the cost for the time you're mostly likely to be using the Net.
Average home use (regular Web surfing, e-mail, and more). Set per-month fee but make sure it is flexible enough to match your (mostly likely changing) needs. Look for cheap after-hours rates.
Confirmed Netaholics and enthusiastic beginners. Flat 'all you can surf' fee but the best bet is to choose an ISP offering several payment plans; you can always drop back to another plan after the initial buzz wears off. (Most Net newcomers rack up huge hours in their first weeks of discovering the Internet, after which things settle down to a less frantic pace.)