I propose implementing a monthly credit subscription. Something along the lines of paying for X amount of credits and then getting an identical amount for free every so often.
ie. If you pay $15 a month for 700 credits, every 3 months you stay subscribed you would also get an additional 700 credits for free. (amount of time to get the free credits can be changed)
This would encourage us to pay a steady amount as it would provide incentive and would get you a steady revenue stream from the subscribers.
I know you can set up recurring payments in Paypal, as I currently pay a monthly fee to support a podcast I enjoy.
Another idea would be if you did a monthly payment of say $15, and get 700 credits every month you would also get an additional 10% free, so in this case you would get an additional 70 credits for being a subscriber. If you also included throwing in the matching free 700 credits every three months it would seriously encourage subscribers.