I am hoping to move for December next year (2014) so that my daugther can start school over there (into her reception year).
We already have started a moving budget and found the catchment area we are hoping to live in. So I would say we've been planning for close to 2 years. Hate for my life to be living for the future, but we both have been looking forward to moving back for years now! 
I am in the same boat as you DTG, we both hate the living for the future thing as well, but we know it will be better for them to grow up with easy access to different cultures and away from all the ignorance and bigotry that the Deep South has to offer [although we live in a little haven in Decatur, Ga].
My eldest daughter is set to start school next year, and her sister the year following [our moving year], but we've decided to keep them out and homeschool for the next few years. Probably until they are 8 and 9, because I want to settle in a bit before we try to navigate all that mess. There is no way on god's green earth I would put them in school here. Even though their local elementary, middle, and high schools are good for our area, the school board has been recently SACKED because of horrible mismanagement, and JUST LAST WEEK there was a school shooting at another school in our country. I wish I could say it was FAR away on the other side, but it was really on about 4.5 miles away.
As for planning ahead, we've set an aggressive budget [$1000 or more a month put aside], we've already got a basic plan about what stays and what goes, and we've already got our inventory recorded. We are planning to ship our stuff ourselves out of Charleston in a 20ft container, and we decided it would be fun to immigrate on a ship--and cheaper, too, with the Dog--so we are taking the Queen Mary II.
Reconnaissance mission scheduled for April 2014 [also his hub's BFF is getting married], and we are very excited as we haven't been back to the UK since the little one was a baby--almost three years now! I'm happy that this time we will be able to do whatever we like as we are planning so far in advance that we will actually have $$$.