Research the school carefully. Be certain they will want to be there. They should spend a half day (including lunch) at each school they are considering. Spend 4 or 5 days in the city before you all make the choices for home and school. Give them control over these choices.
Don't tell them they'll adjust or like where you're moving to. This denies their current feelings. Right now, they don't want to like it. That would be disloyal to their friends if they did.
Drive around. Explore the city. (Get advice on where to go first). Plan a visit back to their old city and friends for the next holiday. Set up skype so they can keep visually connected with their friends.
Material things will not replace the deep loss they will feel so don't tell them they should be grateful for what you have done for them. (After all, you are the one who made them move right?) Making new friends, being connected and involved in activities will keep them busy during the first painful year of adjustment to this loss. Give them opportunity at home to voice their feelings and new experiences. Weekly sessions with the guidance counselor or or family therapist could be a sounding board and give helpful advice for the whole family. Don't just stand by and hope they will adjust. Hug them and tell them how much you love them every day.
They will feel better next year.
The best approach we've found is getting them invested in the process from the beginning, soliciting their input about the home, its location, their rooms, schools, neighborhood etc. The more they are engaged the easier it becomes.
Consider having them bring a friend with them to visit the area and reassuring them that they can communicate and even visit each other may also help to bridge the gap. Our best advice it to focus on not allowing it to become a battle but an essential experience that is going to be difficult for everyone...not just them.