I just had some aggression issues of my own not too long ago, so I understand how distressing it can be. I moved and brought my Apache (sulfur head) peacock with me, I had to wait over a month before I had the opportunity to get more Cichlids close to his size so he had the tank to himself for a bit too long. In hindsight I should have taken my chances and gotten juveniles but I wanted to try an all male peacock tank and many peacocks are hard to sex when they are young.
Anyway, log story short I found a few adults that fit the bill and introduced them and set off World War 3. The Apache was being nasty (as I had expected) so I used the time-out method and that worked for the Apache but the Acei that took his place as Alpha was not any better, I lost an expensive Red Empress male in the fighting (it will be last time I spend $35 on one fish, he was stunning though) and also lost a lot of peace of mind.
It was getting to the point that I was starting to wonder why I bother keeping African Cichlids. I then decided to try getting some smaller juvenile Cichlids and give up on the all peacock dream (I can't find them unless I go to Denver anyway). So I got two yellow labs and two jewel Cichlids and put them in, and the targeted aggression started to calm down quite a bit. The Acei and the Apache still don't like each other but they coexist and so does everyone else.
Lots of plants (live or otherwise, mine are plastic) can also help, I had a tall plant sitting right in the middle of the tank for months to help block line-of-sight and lots of others on the sides for the less dominant fish to conceal themselves in when the boss is being mean.
The smaller fish are great for distracting the alpha, he'll chase them just because he can but it's half hearted because they are too small to be a threat. As several people above have noted, the more Cichlids you can add, the better the aggression will be.
However, I've found there are some individual fish that are downright nasty and will target a specific fish to the exclusion of all others until they kill that fish and then they move onto another target... I don't keep fish if they get like that (I've had very few like that and most I was able to re-home).
Anyway, over the years I've found the best way to have a peaceful Cichlid community is to get all your fish at the same time and when they are really young so they grow up together. It's not always an option, but if you have the opportunity, it almost always works out better that way.
