Bit by bit
Definition 1 of 2
Pro Tip 1/3Persistence Emphasis
Use 'bit by bit' to stress that continuous effort leads to success. 
She mastered the piano bit by bit.
Pro Tip 2/3Time Progression
'Bit by bit' implies that something takes considerable time to complete. 
The city was rebuilt bit by bit after the earthquake.
Pro Tip 3/3Overcoming Difficulty
Use it when describing a challenging task that's completed in stages. 
He overcame his fear of public speaking bit by bit.