That's what my roomy suggested as we were chatting about the Obama/Clinton SC vote, and the primaries, in general. I couldn't think of why they all shouldn't be held on the same day, except that the incumbent candidate - in this case, Clinton (even if she's not officially an incumbent) - would always win.
Anyhow, got pointed to a couple of orgs that at first glance seem to be at least semi-legit:
The Fix the Primaries site lists some possible solutions:
The American Plan
Starting with small states and working towards large ones, the American Plan also incorporates random order to afford big states the chance to go early as well.
The National Plan
This plan calls for a national primary where voters can vote once between January and June and ballots are counted and tallied at the start of each month.
The Delaware Plan
This plan relies on "backloading" the primary schedule, that is, allowing less populated states to go first and the most populated to go last.
Interregional Primary Plan
Six groups of primaries or caucuses would be scheduled between March and June. On each date, a state or group of smaller states from one of six geographic regions of the country would go together.
Rotating Regional Plan
Under the proposal, the country is divided into four regions - Northeast, Midwest, West, and South, which take their turns voting first, then one region per month from March to June.
Regional Lottery Plan
State order would be decided by lottery on New Year's Day. Two small states would be randomly selected to go first, followed by four regions also determined randomly.
One Day National Primary
This plan simply calls for primaries and caucuses in all states on the same day.
The Texas Plan
States are divided into four rotating groups with equal number of both electoral votes and total number of states per each group to provide an equal number of predominantly Republican states and predominantly Democratic states.