I've moved something like 24 times, and I'm getting ready to move again. Most of those were when my boys were small and I was still married to a military guy. Now my boys are going off to college!
When they were preschool age, a move didn't rattle them in the least. They were always excited about it. It was changing schools that bothered them. I think the longest time they ever spent in any school was *almost* 2 years.
Basically, the younger they are, the easier they adapt. When they reach teenage years, they tend to resent a move much more deeply. Friendships in the teen years are very important. When they have to move, they feel they are missing out on everything while they are trying to fit in with a new crowd who doesn't accept them yet.
Email is fantastic for keeping in touch, but watch out for cell phone bills. My boys have friends who still keep in touch from middle school. Now that they are driving and have cars of their own, they visit from time to time. Watch for special friendships and give them the opportunity - even encourage them - to keep in touch with those kids. It will mean a lot to them not only in keeping the friends, but in knowing that you care about it and them.
Something I've tried to do, even if I wasn't always successful, is to understand that what is important to them is valid. Just because leaving friends or a school doesn't upset you for your kids doesn't mean that they aren't going to be devastated, and rightly so. Their world is different, but just as important as yours.
As far as making the actual move easier on them regardless of their ages, make sure that their things are easy to find when you arrive in your new home. The sooner they can pull out their favorite toys and set up their beds with familiar blankets the better. With teenagers, it's the same. With familiar posters on their new walls and their favorite cd playing, it won't be quite as foreign to them. While adults are busy running around with boxes, kids can get lost in the shuffle. It can be scary for them. They need to have their hands on something they are comfortable with.
My older son just came home from work and I posed this question to him. He says that the best thing we did for them is taking them to the new place ahead of time so they could meet people and get used to the town. Going to the school so they could meet their teachers and the kids in class before the move was a good thing too. That way they aren't bombarded with everything being 100% new all at once.