Remembering complex derivation is somewhat like remembering the path to a destination.
First make sure from where to start and where you want to reach.
That is, make note of the first step/technique/formula and have a look at the final step/result.
Then go through the derviations and note the important milestones, curves, deviations etc. You need not have to remember the whole steps, but the start, important curves, deviations and the final result.
Try to identify those curves and turns and concentrate to reach those one by one reducing the garbage data you store, workout the intermediate steps yourselves, you can excel in derivations.