Check valves. You can purchase the quarter turn valves for your manifold with or without check valves. Most hardware stores sell the 1/4" valves in the air compressor accessory section, those don't have check valves.
Once my kegs are properly carbed, I turn off the pressure at the manifold by shutting off the valve. I leave everything connected, and just bump the pressure when the keg needs more CO2.
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