Chris and I have only been married for two months now, but we've been together for 7 years.
We are a lot like fire and ice, the whole opposites attract thing. He's passive and can remain calm in arguments while I am agressive and have a temper. He's quite, I'm outgoing and bubbly. So, one of the things that we do to avoid arguments and make things smooth is to stick to what we know. If we go to a car dealership (for example) I'm the hardass who deals with the salesman. If we are going somewhere to confront someone (for example a neighbor we are having a problem with) Chris takes the lead because he keeps his cool. It gives our relationship an interesting dynamic and I think makes us both feel like we contribute equally.
We also do the arguments over email thing. It works out great because I can form my thoughts much better in writing and gives us both simmer down time. It really started because of how our relationship started. While we both went to the same high school we got to know each other over IM and email, and it has really stuck with us.
Another thing we do is have specified date nights. Like I mentioned, we just got married very recently, specifically on New Year's Eve. It was a full moon this year, so we promised ourselves that every time there is a full moon we will have a date night. Whether it be staying in watching a movie or going out somewhere we set that time aside to just enjoy each other.