Quote Originally Posted by Darth Nihilus
I always get my reputation very high just by not betraying my allies, or atleast notifying them of my intentions rather than just attacking them all of a sudden. I will cancel the alliance first.
In the couple campaigns I have played (M and H campaign difficulty), my reputation tended to stay at "Mixed" even though I did not break any alliances and did not attack anybody without being attacked first. It started to improve (rapidly) only when I executed the mentioned "joint" attacks with my Byzantine allies.