god I hate this kanji sometimes... the different forms are all so hard to keep in mind when it's used as a primitive I hope there won't be more like this later in the book.

並、衣、長 are the three most difficult primitives I can think of. 並 is, in my opinion, a little harder, but less common.　長 is about the same, but again less common than 衣. Master 衣 and you'll be in good shape.

Post-RTK, I mostly remember primitives as above in a kind of kinetic way, flowing and stiffening strokes that get tied into how the kanji's used when I was learning it in words.

Well, while we're venting, allow me to say that I hate those symmetrical primitives/kanjis, here are a few examples of the kanjis I'm always confused with: 丁千干午夫牛半羊失平矢先朱, and don't forget the "scissors" primitive.

A little help that I've found to remember those: they are not strictly "simple primitives" to remember, they have stories of their own too! For example, the "scissors" primitive is a husband using a pair of scissors.

Split it up into a tophat and dress. Sometimes you use the top hat, sometimes you dont, try mould that into your story. Maybe its a sunny day? I dont know.. my stories usually justify no hat with something like "but here you don't need a top hat because so and so," I cant give you a story off the top of my head sorry.

Heisig gives his own names to these primitives as well.
The top hat comes above something and the scarf below, that's different than a garment. If you use those names, I've found I don't get confused.