Can anyone recommend a good technique on microphone grounding and elimination of 50 Hz noise?
This is a simple headphone mic. When I touch mic's body the noise is decreased significally.
To confirm that the body of your mic is grounded properly you can use an ohm multimeter and measure between the body and pin 1 of the mic. If it shows more than 0 ohms then you have a grounding problem with your mic.
You should also try swapping out mic cables and, if possible, try a different preamp to see if that's where the issue lies.
If you are unable to remove the noise via analog means, you can use a noise reduction plugin. Many plugins have 50hz, 60hz, etc. presets built in, but you can also record in the noise and the plugin will subtract that frequency component from the signal.
Otherwise, the ghetto way to do it is to use a notch-filter at 50hz.
Run all of your equipment from a common ground (simplest way: string everything off a single multiway adapter, although be careful of the current draw on the poor socket and your house's RCDs and building ringmain if you have multiple 13A devices!).
If you have to cross power and audio cables, do so at right angles. NEVER run power and audio cables in parallel.
Use balanced connections wherever possible; XLR microphones and monitors from a balanced mixer or amp is desirable. Balanced TRS is less desirable but 'works' although you may still get some noise ingress.
Check the wiring of your microphone if you're getting buzz / hum until you touch the body / capsule. You mentioned the mic is a headset mic; you may have an easier time just ditching it and buying a newer/better one.