The Civil War proved that succession is no longer allowed.
All hail the federal government, whether you want to or not.
While I personally don't think that ANY state will seriously attempt to secede in this day and age I also don't think that we would ever again have a civil war to stop them. Hell, we (the US/Federal Government) couldn't afford either the actual war or to rebuild several states after we blow them up. How much you reckon it would cost to rebuild the 4th, 7th, 9th, 14th, 16th, and 19th largest cities in the entire US and that is just in the state of Texas. When you include the massive economical hit of any such war, its just not going to happen.
And those aren't even close to the only issues. Lets say Louisiana and Texas secede and the US tries to use force against them. Those two states alone can and most likely would immediately cripple the entire country's economy by simply shutting off their supply of go-go juice. For everyone outside of those two states it wouldn't matter how much gas cost because there wouldn't have any to sell in the first place.
At the end of the day, its a fun conversation to have it is also an irrelevant one because no state is going to secede, period.