Different states have different rules but in some states you can just put a new breaker in your box that connects the solar output to the rest of your house and a mechanical interlock (literally a stamped piece of metal) that prevents both the main breaker and solar breaker being closed at the same time.

Ah, OK, so that would work for the "simple" systems, but not the ones where you can feed power back into the grid and get a credit.

For those you need an automatic transfer switch (ATS), such as the Enphase Enpower. Such devices contain circuitry to automatically manage disconnecting your home from the grid, so you 1) maintain power in the home (at least on the critical circuits you decide on), and 2) don't island your home (so linemen don't die).