Republicans in Mississippi, North Carolina and Indiana (all open primaries) should cross over and vote for Obama. This would have the affect of helping him grow his lead over the Hildebeast. Then, when she steals it at the convention there will be more outrage than if the leader was closer.
Another thing that Republicans can do when they are voting in the Dem primaries is to vote AGAINST all Dem incumbents in the House and Senate. Maybe we can get a few of the knocked out there and would strengthen the GOP candidates chances in the general election.
A gap that is too big in favor of Obama will not do any good, since it will make it less likely for Clinton to steal the nomination. Vote against Dim incumbents are good idea.