My advice is to have more than one approach - use many methods. Here are a bunch:
Say the lines, repeat them.
Record them on your phone and then listen through headphones as you walk, wash up or commute.
At home, listen to the recording and do strong gestures on each line or word.
Edward de Bono advises making things vivid to make them memorable.
Look at the techniques Derren Brown gives in his book 'Tricks of the Mind'. The technique 'loci' is very interesting and may be adaptable to your purpose / particular script..
Action your lines (see Mike Alfreds' excellent book 'Different Every Night').
I look for patterns in consonants. Make strong visual connections between last word in one sentence and the start of the next.
Mike Alfreds says: if there's a line you can't remember, give yourself (i.e. your character) a strong want to say that line.
Vary your activities - be kind to yourself don't make it tedious.
Sometimes sit and study text. Look at the logic of why each sentence is where it is, look at the argument developing, or the steps of the temptation/of the reveal/of the escalation of the argument.
Divide text into 'units' see Mike's book. Read a unit, put down book. Do from heart/pr improv the sense, then see how the actual text is again. redo. Move on to next unit.
Sing the lines - use all yr senses! Sing and dance.
If it's a tricky dialogue, consider learning both sets of lines - that's what Maya Angelou did.
Learn short chunks and repeat while doing something else - eg washing up.
Try this: sit and breathe. Sit and look at a page, breathing. aim to softly see whole page. apparently imagining to have put something on the back of your head - where Jewish men wear the yarmulke - while you do this (stimulates a part of the brain). A tip from Paul Schiele's course 'Photoreading'.
Sometimes just do each line 'parrot fashion' the way you learned your times tables as a kid.
Congratulate yourself often - practice self compassion.
Great TED talk on self compassion.