You have to do this BEFORE you feel anything negative. Otherwise you don't want to do it. So look for signs where you guys are starting a debate. Ask to hold hands or sit on their lap or be hugged the whole time.  It helps simmer down the situation being physically comforted/reminded that they still care and since it feels good to be in their arms, it reminds you that YOU still care about them too. 

Another tip: Make a rule to take turns talking, without interruption. After one finishes their turn, take 10-30 seconds staring into each other's eyes and have a moment of silence. It's basically like deep breathing, but for arguing.