I believe there are many factors that make an LDR successful. I also believe that all couples are different and are compatible in different ways. What worked best for us may not work best for others.
The two main things for DH and me - communication and trust. We have been completely committed to each other from the very beginning and have trusted each other implicitly. We spoke on the phone everyday (and still managed to have wonderful hour long conversations on the weekends), e-mailed, and sent letters and cards. We openly told each other how much we loved and missed the other.
We definitely got frustrated along the way though! Patience is not one of my virtues!
It helped to set dates for visits and we tried to plan ahead as much as possible. I feel lucky because we were able to spend four months living together one summer after about a year of LDR. We learned a lot about each other's living habits, values, future goals, etc. and had lots of important conversations. It confirmed for us that we were definitely meant for each other. It was a lot harder to go on after that, but at least we knew we were doing the right thing.
I've only been married 5 days, but so far everything is great! haha.
