Start with a strong simple phrase
A short melody with a clear rhythm is easier to remember and arrange than a constant stream of notes. Hum the idea without the instrument to test whether its shape is memorable.
Try changing the final note or leaving a gap before the phrase repeats. Tiny differences can make a loop feel more human and intentional.
Add call and response
Let a second sound answer the main melody rather than playing continuously alongside it. This creates conversation and leaves room for the drums or future vocals.
Use a different register, tone or rhythm for the response. Keep the supporting phrase quieter or shorter so it does not compete with the lead.
Automate arrangement changes
Mute an element for a few bars, change an octave, filter a layer or add a short transition before a new section.
Save the strongest variations for moments where the song needs renewed attention. If every bar changes, the listener may have no stable idea to hold onto.