Term for doing something little by little?
Solution 1:
The review for the exam was 'piecemeal' (adv. and adj.):
- By a small amount at a time; in stages: articles acquired piecemeal.
[piecemeal. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved January 20 2016 from http://www.thefreedictionary.com/piecemeal . Emphasis mine.]
For example,
Her review for the exam was piecemeal.
'Piecemeal' communicates the chronological dimension of what I understand to be the meaning you want.
Solution 2:
Incrementally
Incremental, according to Merriam Webster:
of, relating to, being, or occurring in especially small increments [e.g.] incremental additions
Vocabulary.com says:
If you are making incremental progress in math, you are moving slowly but steadily forward. Incremental describes regular, measurable movements that are usually small
Incrementally is doing things by increments. In your case you are reviewing your course-work incrementally, in easy to absorb bite-size portions.
Doing this for a week is better in the long run than cramming for hours the night before an exam; you will retain more.
Solution 3:
Gradually
According to Merriam Webster gradual is defined as (for this meaning):
1: proceeding by steps or degrees
2: moving, changing, or developing by fine or often imperceptible degrees
Vocabulary.com (for this meaning) words it a bit differently:
proceeding in small stages
Gradually is often used to describe things that are done over time, in relatively small amounts each time.
Solution 4:
Pace
Google Definition:
Verb
do something at a slow and steady rate or speed in order to avoid overexerting oneself.
Rather than cramming for an exam, a student should pace her studies throughout the semester.
Update in response to downvote.
While the definition of the word does not exactly fit the OP's word "little by little," I feel this word is very commonly used in the above situation by students:
Hey, what are you trying to do? Memorize the entire book in the first week of class? Pace yourself!
(while cramming) I'm not going to put myself through this again. I'm going to pace myself next semester.