All good suggestions, you should also have a merc running around looting if you can. The AI is much better about pursuing looters now, but two or three high tax provinces pay for the merc anyway. Another good trick is to land in a province after it's 75% done building a troop, the AI has paid for it, but cancels it anyway. Looters can thus eat the economy of your enemy. Looted reduces a provinces income by half for 6 months, and the money they used to make the troop is gone forever.
Looting gets you the base tax of a province in ducats directly to your coffers. The AI will almost always pursue a looter that goes into a province bordering their stack, and since 1.7 will chase you from even further than that if they've nothing better to do (from the AI perspective).
You can't loot provinces that border any province you have access to (even military access), except the HRE access doesn't count.