It depends a bit on where you are, ie Tropics
Moisture/Mould is a pain if you can't get air through the boat for 6 months.
Tip I was given was to leave an open container of Bleach in each cabin/head. Keeps the mould away.

General knowledge is to leave your tanks full so that condensation is minimized. But then I wonder after 6 months how 'good' the fuel is?
And it can cost a bit to full it!
I'd add bleach to your water tank and on return flush it out for new water. (One cap full for 200-300 litres)

Disconnect the fridge and leave the lid open.
If you leave the boat on solar power you could look at connecting some fans up to a timer or switch that will run them for a few hours. That way you keep the batteries cycling.

Flush the outboard and spray it all over with WD40.
Make sure the dinghy can drain out. Don't want it full of water the whole time.

Spray zips, locks, hinges etc with plenty of CRC. Definitely lubricate all sea-cock levers.
Protect/cover any electrical cables that are left exposed. For example, connections for the Auto pilot, plotter, sounder etc (if you have the types that you can remove and hide away.

Put away you good sunbrella/canvas. Why let it get 6 months of sun/weather on it when your not there. Buy some cheap tarps for covers and throw them away when you get back. They will only last that long.

Cheers