This kind of stuff is usually set out in constitutions or basic laws which in turn can be enforced by courts – the usual division of power stuff. You could also introduce some sort of check on the power of a global government. In Germany, parliament has to get approval of the majority of state governments for certain laws to enter into force, for example. Same is true for the E.U.: The E.U. parliament and the council of the national governments of E.U. member states both have to agree to new laws and regulations in most cases, which guarantees that either one can't ride roughshod over the core interests of the other.
It usually is, but long-term there's a trend of, shall we say, creatively reinterpreting what's written to the same effect - just look at US. Some would argue that EU shows the same trend, although it's young enough that this doesn't manifest quite so much.
I’d actually argue that the increasing centralization of power in the U.S. is due to the increasing nationalization of political issues and the requirement for nation-wide resolutions to problems. The way it has expressed itself (strong presidential executive instead of parliamentary democracy) is probably due to the way the U.S. constitution was originally framed and some random events asking the way.
But that's the thing - why are these all suddenly "national issues"? Even healthcare can be done state by state (Canada of all places did it that way, and their system is still fundamentally province-centric), never mind all the culture war stuff. Is it really the most local level on which these matters need to be resolved? Or are they deliberately pushed there for political games?
