What are the good ways to maintain the bicycle while raining season?
Use fenders with good coverage! They will keep most of the dirty spray water off your bike. You'll be surprised how much less oiling your chain needs with proper fenders.

Proper fenders means:


a front fender with a mud flap that reaches within a few centimeters of the road
a rear fender that starts some centimeters below the chain stays, so that water dripping off does not end up on the chain