Take your disc rotors off unless you want them buckled!
Shim your brake pads to stop pistons popping out.
Remove your rear mech too - I take the hanger off with the mech so there's no chance of it getting bent.
Pedals off.
I take my bars off and zip tie them to my forks.
Another tip I do after seeing bike bags walked on - I replace the rear skewer in the frame and use a 135mm tube (copper pipe) to act as a dummy hub and stop any crimping of the dropouts.
Regarding air pressure - 1 atmosphere is only 15psi so no need to let the tyres or shock down. If the check in staff ask you to do so, ask them if they let the tyres down on the plane! (just have a standby ticket on another airline incase they get the hump

)