Just as a good mutual fund never has more than 5% of its total asset value in a single security, no single state should control more than 5% of the electoral college.
Therefore, any state with more than 5% of the total number of electoral votes (as of this writing, 538) shall be subdivided (solely for the purposes of the election) in such a manner that: each subdivision has about the same population (i.e., each subdivision shall be within 1 vote of the other subdivision for that state); the total of the subdivisions is equal to the amount for the state as a whole (no more, no less); each subdivision's electoral tally is less than 5% of the total for the United States; and the subdivisions must respect existing city, county, and/or natural boundaries.
E.g., FL split in 2: 14 and 15 votes.