Having goals have been the best help for us. When we got into this, we knew we'd have at least 4 years of a wait, if not more, to be together, because I had just started college and I wasn't getting married or moving until I was done with school. Because of my school commitment, and his job commitments, we only have time to see each other 2-3 times a year. However, after each visit was over, we had a new visit to look forward to, and the big end is in sight now -- I graduate in May, and we're getting married at the end of October. It has been a rough four years, and we have had our share of laughter and tears, and like everyone else has said--communication is a big part, as is trust. Support from those around you is, too, because sometimes you'll get the skeptic or the person who huffs at your situation, and sometimes that can get to you (it has to me in the past at least).
We keep all sorts of communication going. We use IM, voice chat, video chat, phone calls, texts, emails, and occassional letters and cards for occassions like birthdays/anniversaries that we miss from being apart. We spend as much time together as we can when we're phsyically together and try not to take it for granted, whether it's been only 2 weeks, or 3 months. In the past 3 1/2 years of our relationship, we've only spent 1 year physically together in total but we're still strong. I don't think there's a hidden secret to it all, but you need to find what works for you, and just realize that things won't be easy...but don't give up because things are hard!