I would add that it is important to play through complete games in order to also see typical endgame or late middlegame strategies. Usually in the Najdorf, Black's position in the endgame has some typical trumps like the Bishop pair and the K-side pawn majority. Many of your wins will come in the endgame and not knowing how to use these trumps will be a problem..
And some (more) ways to help you remembering variations:
- In the beginning forget a bit about concrete variations and try to get a feeling of the opening by reading about it and also playing friendly games (or vs. a computer).
- Every now and then play some blitz games and then check to see where you deviated from the book and why. Experience (especially if painful) helps to get the moves stuck in your head.
- In very tactical variations like 6. Bg5 (as Alias pointed above), where you have no big room for deviations and some common sense moves may be losing on the spot, it is a good idea to understand the concept behind each & every move (I find this is the easiest way for me to memorise difficult lines). It is like remembering a story, e.g.: My Queen goes to c7 and thus stops Bc4 or it covers b7 and I can now safely play Bd7 without fearing e5, etc.
And some practical tips that may be your deal or not (read these with an open mind):
- Be practical, choose similar variations or ways of playing whenever possible, so to cut down on therory. You can expand your knowledge later on.
- Avoid the poisoned pawn variation for the moment, it comes from another planet.
- Save some of your opening study time to reinforce your anti-sicilian systems (you will face anti-s a lot and they are equally dangerous).
- ..and take baby steps - one chapter at a time!
my 2c
Edit: I should add there are other ways to memorise opening lines by using specialised software, e.g. Chess Opening Wizard. If you are interested I can point you on some threads here. But I believe first you have to go the traditional way, i.e. read the book, practice and take notes..