Chicago recently privatized its parking meters, ripping out the old coin-operated ones that stood at each space with a smaller number of centralized, coin-and-credit card-fed boxes that print out receipts to put in your windshield. I think they could have done a much better job:
First, instead of paper receipts, regular parkers should have little RFID badges, perhaps that plug into cigarette lighters to recharge. This way, meter maids can just walk down a street and tell without even looking if a car has gone over its limit.
But the real benefit of the RFID badges would be that they'd make for a much more interesting, targeted payment system. For one thing, you could enable people to pay for parking remotely. Instead of having to walk to their cars to feed the meter every 2 hours, they could do it online or over the phone.
But part of the reason you wouldn't want this functionality would be that you don't want people to hog spots all day. So, in exchange, you'd raise the prices, perhaps substantially. (They should probably be generally much higher than they are now anyway). That would both raise more money, and enhance the quality of life for people who frequently don't have any choice but to park in a metered spot for a long time.
Now, this type of thing often has the unfortunate side effect of making life a lot more expensive for the poor and lower-middle class, so you could create a variable pricing system tied in part to individual income, deduced from the state income tax receipts. You could also make the pricing somewhat dynamic, based on demand.
This idea has a number of potential pitfalls, but it would a) raise more money; b) make parking much more convenient for people; c) be much better calibrated to individual income levels; d) make enforcement much easier and e) generate boatloads of interesting and useful data about who parks where and when.