1. Pick your battles (as has been said by PPs in different words). Some things just are too small and unimportant to let them come between you.
2. Another vote for throwing out the "Don't go to bed angry rule." DH and I rarely argue, but when we do we've found a cooling-off period (especially an overnight one) allows us to see things more clearly w/o being clouded by high emotions. One or the other of us usually apologizes first thing in the morning, and it's helped by...
3. No name calling. We take care not to let any anger be put into hurtful words.
4. Think of your marriage as a third entity that's more important that either of you singularly. I read this in a self-help/marriage book some time ago, and it was like a light bulb went off in my head. He's not more important than me, I'm not more important than him, but our marriage is more important that either of us. This rule of thumb also helped me leave a previous bad marriage when I realized that I was following this rule but ex was always taking care of himself first.
5. No secrets. You should always be totally honest with your spouse and expect the same from him/her. Other friends and family members know this is our rule, too. Don't tell one of us something and expect us not to tell the other. Keeps anyone else (especially ILs) from driving a wedge between you.
6. Don't belittle one another. This is hard as DH and I sometimes rib one another in an attempt to be humorous, but we've learned over the years not to take things too far. E.g., I can tease him good-naturedly about being thrifty (he makes fun of himself, too!) when it's just the two of us, but never in front of other friends or family members, especially SIL. She tells everyone in the family that he's cheap and she tries to put a humorous spin on it, but it obviously bothers him. It's also rich coming from her, who spent years racking up all kinds of luxuries for herself by using credit cards, then declared bankruptcy and moved in with MIL when she couldn't pay her bills. She tries to draw me in on the "He's so cheap..." bits, but I never play along and I stick up for him.