My least favorite system is the one where there is just a list of everyone's AWs and you go through them one-by-one and donate until they upgrade. There is no way to make sure that people investing a lot of KP get that back in their AWs.
The easiest way that maintains pretty good fairness are swap threads. They take very little oversight and everyone gets back what they put out. A swap thread works by creating a thread for a certain amount (say 5KP to start out). The first person posts what AW they would like to donate to. The next person who wants to swap some KP posts that they donated to the previous person and then lists what AW they want donated to. And on and on. You simply have to start a new thread when too many messages have built up.
You can have as many swap threads as you like, for whatever denominations work for everyone. People can jump in or out at any time, you don't have to have AWs listed in the city name (unless that is in the hopes of neighborly contributions). If two people post at the same time, it can usually be caught quickly and rectified.
The best method is definitely the Wonder Society program referenced above. It takes a lot of babysitting, though, so may not be the way to go to start out with. It ensures very fast leveling and the greatest fairness all around.