I like this reader’s idea when her kids fight:

I’m a mom of three children — two sons, 9 & 12, and a daughter, 16.  My kids have had their days, but for the most part they do get along (then again today’s a good day!).  Some days I admit I let them go when they argue and think, yeah they’ll work it out. But then there are days I’m like “guys, this is hurting my feelings.  I know that you guys have to be hurting as well.”
 So a few years back I decided to try a few things to get my kids to get along.  The first thing I did was to place whoever was fighting in an XXXL shirt and make them wear it together and walk around together and work together.  Then I decided to try something else, and to this day, four years later, it’s working great!  I get an eye-rolling every time I make them do this, but they know they can’t do anything else (computer, iPod, play outside) until it’s completed.
  I make them face each other in a chair, leg-length away (to avoid kicking).  They have to sit there, with no distractions, until they say three nice things to each other (and it can’t be “your hair looks nice,” etc.).  The first few times they did this, it took an hour or longer.  It all depends on how angry they really are, but for the most part they are done quickly and end up laughing.  I have them hug, shake hands and say, “I’m sorry/I forgive you.” 
It teaches them that arguing and fighting are unacceptable behaviors.  It also teaches them that eventually you have to work it out and talk about things.

Relationships start at home. I tell my kids that communication and compromise go a long way and are keys to a good relationship.  I hope this helps them have strong relationships in the future.