First off, you need to centralise your capital. The further a settlement is from the capital, the more corruption there will be. Centralising your capital will really help out.
Second, build buildings, such as temples, which give law bonuses. Bring in governors who add to law. If you have the 1.5 patch (and 1.6 if you have BI), then the Academy and the successive upgrades all give a law bonus, and they'll therefore help cut corruption.
Unfortunately, there is only so much you can do. In the end, you will still end up with corruption. You can only do so much. The measures above are the best, by far.
Also check your family members if they themselves have corruption traits. That can hurt your economy if left unchecked. If you get over 50,000 denarii in the bank, and keep it over that, your family members will start to generate corrupt traits, etc. So try to look at the upkeep of your armies, etc, and try to see if you can keep your banked money below 50,000 denarii. Eventually, though, you'll still end up making more money than you can throw a pilum at.
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\/\/\/\/\/\/\/\This is Bunny roasting on an open fire. Yummy!