First, forget the usual x and y axes — tilt them! Make one axis parallel to the slope and the other perpendicular to it. Gravity's force then gets split into two components down these new lines. I redrew it fifty times last year before it clicked. The thing is, my diagram was perfect, but I forgot the friction force pointing up the slope. In my life, sliding pickle jars off a tilted shelf taught me friction's direction more than the textbook ever did.
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