As one of the most complex
parts of a bike, the drivetrain
needs constant maintenance.
Check chain for wear 
Ensure gears are shifting properly
Inspect cables for fraying or wear
Tighten crankarms and chainring bolts
Oil chain and jockey pulleys if the bike was ridden in the rain 
Check bottom bracket runs smoothly 

Oil chain and jockey pulleys

Tighten pedals, if needed 

Check cog teeth on-chain/cassette ring are
not worn down or missing 
Ensure rear derailleur hangers are fixed 
Spray derailleur hangers, cables, and clipless pedal release
Check for play in freewheel, freehub body, and rear derailleur frame bolt 
Ensure jockey pulleys are not worn down 
Oil hub gear and check pedals do not feel rough or
notchy and are not worn down
Replace chain if used less regularly 
Replace cogs if worn out
Wheels and steering may require
frequent attention if you are riding
more on trails than on roads.
Check headset is correctly adjusted and allows for easy steering.
Check quick-release levers are functioning
Ensure wheels are in true and have no broken spokes
Inspect handlebar and stem for cracks and ensure stem bolts are tightened
Check hubs for any roughness, tight spots, or play on axles
Ensure there are no splits on the rubber hub
Inspect headset covers, if installed 
Oil hub seals 

Inspect bearings in open–bearing hubs for wear
Check for wear in bearings and bearing surfaces
in headsets.

Grease open bearing hubs and headsets
Replace handlebar tapes and grips 

Brakes can prevent all manner of
accidents, so regular maintenance
checks and repairs are crucial.
Inspect inner cables for fraying and outer cables for wear, then oil with lube 
Ensure pads are aligned and not worn down
Tighten disc and caliper bolts.
Check for cracks in brake parts.
Inspect hydraulic hoses for wear or leaks.
Ensure discs are aligned and not worn 
Grease inner cables and oil inside outer cables
Replace brake pads of frequently ridden mountain bikes
Grease brake bosses
Replace inner or outer cables 
Regularly checking suspension
systems can prevent small problems
from developing into larger ones.
Check over fork and shock exterior surfaces for cracks.
Inspect fork stanchions under shock boots for cracks.
Tighten top caps, crown bolts, and shaft bolts.
Lubricate fork stanchions with wet lube.
Eliminate any play in forks and shocks
Check fork stanchions to see if the oil line is visible
Inspect fork and shock seals for cracks or slackness
Ensure there is no fork or shock sag 
Turn the bike upside-down and store overnight so the oil can spread through the fork.
Remove headset to check fork steerer assembly for cracks.
Replace fork oil.
Have suspension maintenance done by a trained technician.