Jimmy Carter and James Baker offer this Roll Call oped (paid subscription required). A snippet:

The Carter-Baker Commission has endorsed the proposal of the National Association of Secretaries of State to create four regional primaries, held after the Iowa caucus and the New Hampshire primary at one-month intervals from March to June. The regions would rotate their position on the calendar every four years. The NASS plan would substantially expand participation in the selection of presidential nominees and give voters the chance to evaluate presidential candidates over a period of three to four months. The commission’s recommendation seeks the change for the 2008 primaries.
We believe that it is important for the parties to maintain control of their own primaries. Therefore, we would encourage the two parties to make the needed changes in their primary schedule. If the parties don’t take action, they risk losing that power to Congress, which should make the desired change through federal legislation if the parties remain unwilling to do so.