I'm sure I'm forgetting a couple resistances that got removed/changed to dark and steel in the last generatios... otherwise I think I remember them pretty well.
I forget resistances more than weaknesses because weaknesses tend to make more sense logically.
One think that helps me nail them down is the a possible reason for that weakness/resistance, especially the less obvious ones.
Fairy types for examples are weak to poison and steel because they are based on a whole lot of mythological creatures and folklore from various cultures: fairies, gnomes, leprechauns, banshee... most of these have myths about them on how to ward them off, the most common ones being wearing something made of iron or various erbs (of which some were poisonous).
I love to read little things like that and if it helps me remember something even better