The trick to which I think Daeron is referring is a way of getting the AI to DOW you. Here's an example: I'm at war with Civ A. I declare on Civ B, then I make peace with Civ A and I include an MA against Civ B in the treaty. The chances of Civ A actually fighting for the full 20 turns against Civ B is pretty slim and when Civ A makes peace with Civ B, they will be forced to DOW me. I think you get War Happiness from this, but I could be wrong on that.
I've never used that one myself. But yes, unless you still have war weariness from the first war with Civ A, you should get war happiness.
For a continents game it probably does require going to war with a neighbour you were planning to go to war with anyway.
If you don´t want to play Machiavelli, the best way to get a war is to ruin their attitude with you, putting in a few demands. And if they ever demand something from you, refuse and hope they declare.