You can't increse the amount of corruption reduced by courthouses/police stations. The best way of further reducing corruption is to make other important improvements (ie. Temple, Library, Cathedral, University) reduce corruption as well. Also, you should make use of the Forbidden Palace small wonder as having a second palace can really help you out if you have a massive, farflung empire (especially when you're not commercial). If you have plenty of Great Leaders to spare at a location far away from the capital (preferably on a different landmass altogether), you should rush build the FP.
