Any state that wants to secede should suffer the following consequences if they do:
1. Require a passport to visit the rest of the U.S.; this would royally screw over their sports teams.
2. Tariffs on goods going to/coming from outside their state.
3. 60 days to set up their own power grid if connected to a national one.
4. No disaster aid.
5. All national credit cards rendered invalid and bank accounts closed within 90 days.
Obviously, their representatives and any other employees in the Federal government would also be kicked out of their jobs. And kiss any benefits such as medicaid, medicare, social security, and welfare goodbye.
Implement those, and see how quick they ask to rejoin the country!