You remove as many barriers to the purchase as possible. Allow monthly credit card billing, but incentivize longer term payments. For example, PivotalTracker.c

You remove as many barriers to the purchase as possible. Allow monthly credit card billing, but incentivize longer term payments. For example, PivotalTracker.com charges monthly. However, I personally paid them for an entire year to get the 2 month discount! 12 months for the price of 10 months is enough encouragement for me to pay for the entire year.