Melody, motifs & variation
A memorable melody is usually one small idea repeated with one meaningful change.
A motif is a small, recognisable piece of melody: a rhythm and contour short enough to remember. Repetition teaches it to the listener; variation keeps listening worthwhile.
Listen for: Constant novelty is hard to remember.
Shape before note names
Think in contour: rise, fall, repeat, leap, return. Scale-degree instruments make this easy because every vertical position belongs to the key. A strong rhythm can make three notes more memorable than an aimless run of sixteen.
Repeat, then change one fact
Repeat the motif exactly once. On the next pass, change only the ending, starting note or rhythm. The listener hears both identity and development because most of the evidence remains familiar.
Range is another form of emphasis. Save the highest note for the moment that deserves it; if every phrase reaches the ceiling, height stops meaning anything.