The biggest thing is to think before you speak. If your spouse says something that upsets you or makes you mad, think about what he or she is saying and why your spouse is saying that. Respond in a normal voice, instead of yelling. If you feel you must yell, take a second and think about your response.
Go on a date night/day. Pick a time when just the two of you can get away alone, even if it's just for a walk in the park. The point is to get away from the stresses of life, and, if you have kids, to get away from the kids for a couple hours. Kids can be a source of stress in a relationship. When you go out, do not talk about your jobs, finances or anything that is a stressor in the relationship. If the kids are a source of stress because you and your spouse do not agree on a certain aspect of raising them, do not talk about the kids.
Make decisions together, even if they are only small decisions for now. You can work up to the bigger stuff that causes arguments. Always take small steps, then work up to the bigger, more important decisions. Something as small as what to make for dinner everyday should be shared so you can learn to work as a team again.