Quick answer
poco a poco
Natural: step by step / gradually
Literal: “little by little”
How it is used
Poco a poco.Little by little.
Voy aprendiendo poco a poco.I am learning gradually.
Poco a poco se hace todo.Everything gets done step by step.
Mejora poco a poco.It is getting better bit by bit.
Where it is said
It is universal and neutral; in Spain you may also hear the fuller saying <b>poco a poco se anda lejos</b>.
A mini dialogue
— ¿Qué tal tu español?— How is your Spanish going?
— Poco a poco, pero voy bien.— Slowly, but I am doing fine.
Watch out
poco a poco
Do not say poco por poco; the fixed preposition is a and never changes.
Check yourself
Which reply means “slowly but surely”?