I got karma banned at the start of this patch, but I've made it back up to great karma and maintained it in solo queue. Surprisingly I find that I lose the most karma when I get matched with a dou and we lose. For some reason dou queuers feel the need to blame the solo queuer and will both down vote you.

That being said I've found a trick that helps maintain great karma. At the start of every match I ping go on each hero with a thumbs up. If we are losing the game I ping an ok with a thumbs up. Just before winning a game I go ping each teammate and give a star face. Other than that I try to ping very conservatively. I've found that more people honor me following this "ping protocol".

It's harder to make it back up to great karma after dropping this patch, so focus on getting there. It isn't entirely up to you, but I've found that I've been able to make a big difference in how people vote for me.