I was in a long-distance relationship for three years and one of those years was spent with my boyfriend on another continent. We were long distance from the start, since we met online, but we made it through and are engaged and living in the same city now. Looking back, it sometimes seems like it wasn't that bad, but make no mistake, a long-distance relationship is incredibly difficult and requires a high level of dedication and maturity on both parts.
It will not be fun
You may love the person, but this isn't going to be a lot of romance and dates. You have to get used to the fact that your level of commitment to the other person has to be much higher than might be normally expected in a dating relationship. You have to trust them, be able to talk to them honestly, and be willing to make time for them (which is even harder when the other person lives in a vastly different time zone). You will have to let things go. You can't let fights fester, because the other person isn't going to be there to remind you how much you love them with a hug or a sorrowful look. You both have to slog through the problems and work shit out. There will be lots of angst.
Get a computer
I spent a lot of money on phone calls until we started using voice chat programs to talk via computer. It revolutionized things. We could talk as long as we wanted and it was invaluable in avoiding international phone fees. We'd always talked via ICQ and AIM before, but having free phone calls helped us stay in touch much better than by just chatting online.
Know when it will end
It's not important that the end of the relationship be marriage, but you should both have a clear idea when the separation will be over and what both of your levels of commitment are. My boyfriend and I knew that we had four years to get through while I finished college. We managed to cut a year off of that, but having an idea of when the end was, even though it was far away, always helped. If you don't know when you will be back together again (for good, not just for a visit) then it will be much harder to keep going.
Sex will be weird
If you're in a sexual relationship, that's obviously another factor that gets distorted by distance. The longest we were apart at one time was seven months and when I saw him again I hardly recognized him. It was hard because he would want a lot of sex and I wouldn't, having basically ignored my sex drive for months on end. But whatever the situation, having sex again with someone you haven't seen for a while can be strange.
Reuniting will be hard
It was probably harder than it needed to be for me, since when my boyfriend and I reunited we were also going through reverse culture shock from both having lived for the past year or so in another country. But even though being in the same place again seems like it will solve all your problems, it won't. You both will have changed and won't be used to having the other in your daily life. You won't be used to interacting with each other and physical affection will take some time to work out. After three years apart, it took my boyfriend and I four or five months to get comfortable with being together again and work out how being together changed our lives.
Having a long-distance relationship required an extraordinary amount of commitment, trust, and slogging through bucketfuls of angst. I got engaged part way through and actually it didn't really come as a surprise. Both of us had put such a level of stubborn determination into making it work that it felt like the commitment to getting married couldn't possibly be harder. In a way, we'd both decided on a marriage-like attitude towards each other from maybe halfway through, simply because that's what it took for us to keep it together.
The best thing to remember though is that it can be done! It's hard and will force you both to go through a lot of pain, but you can get though and it and your relationship will be the stronger for it.