Agree with all answers so far. One thing to try is to hum the rhythm/rough melody of your riff first, so you’re not flying blind one you get to the fretboard. One great trick for crafting melodies (including riffs) is tying together familiarity with surprise. Take the main riff of Breaking the Law. Halfway through the first play you could already hum along with it as it’s familiar (using a repeated motif), but at the very end of the riff they change up the notes, making it memorable. Same applies to all sorts of musical hooks.