I've found that if you attack the enemy before they attack you, take just one or two cities and kill the majority of their army in the region, they will come to you wanting a ceise-fire. If you accept (even though you will usually have to give up on taking one of their cities, mid-siege), their 'hate-gadge' will reset to zero. Then when you start the war again, you can do the same and fight your wars in stages. Of course, this doesn't work if they attacked you or they have no other enemies. Battles seem more desicive this way.