Maintenance for the M2 can be broken down into three main categories, based on required frequency: per print, once per week (or after 50 hours of printing), once per month (or after 200-250 hours of printing).
Observe first layer for signs of unlevel bed, and inspect bed surface
Check that the bed is level with the normal procedure (raise bed to within 1mm of nozzle, move bed forward and back while observing gap, adjust if necessary; return nozzle to center of bed and move left and right, observing gap, and again, adjust if necessary)
Check that the Z endstop activates at the correct location - ~75% of first layer height, or 0.15mm
Check that the bed print surface is free of defects (torn or bubbled tape, etc.) and dust/dirt/grease; replace tape or clean surface with appropriate cleaner (rubbing alcohol diluted to 50%/50% water recommended)
Check that the X and Y belts are the correct tension - with the bed or extruder at the opposite end of travel from the motor, pinch the belt together ~1 inch (25mm) from the edge of the motor drive pulley; it should be possible to pinch it completely together, but difficult
Check that the fans and fan guards are free of dust and filament debris, and clean if necessary
Clean the X and Y linear rails, and the Z leadscrew, of grease, then apply a fresh coat to each - a dot of white lithium grease in each of the long grooves on the X and Y rails, and a dot in four consecutive troughs of the Z leadscrew; once applied, move that axis through its full travel multiple times to spread the grease. If you have one of the newer black Z motors, which has a plastic nut on the lead-screw do not apply grease. Check for and remove any built up up dust or other contaminants.
Clean and wipe the Z axis rods with 3 in 1 oil.
Check the filament surface and filament drive for dust, and clean if necessary.
Check the entire frame, especially the bottom corners, for dust, and clean if necessary Y linear rails.
Check for wear any place filament contacts regularly: The guide above the spool, the top of the filament drive, the drive gear and the nozzle.