If the country you want to DOW has a CoT, flood it with your merchants. Not all countries are as likely to embargo you, but a lot of them will (Venice, especially, seems allergic to human merchants in it's CoT).
I find the loan them money thing to be quite difficult to pull off. Haven't tried it very often, but I don't remember the AI ever accepting a loan, even with a 1% interest rate and a zillion years to pay it back.
Warning a nation (or, alternatively, guaranteeing the independence of a nation that might get attacked by your enemy) is also a possibility, but, again, the AI isn't suckered into wars by this most of the time. At least, that's my impression.
You can also arrange your alliance in such a way that you're bound to get into wars, either because your alliance members are warmongering SOBs or because they always seem to get picked on by others. Either way, you got a nice CB.