Practice ascending long “filled” octaves (C-G-C/4-2-1), using the knuckle of the second finger to muffle the still-vibrating previous middle string as you go. Instead of trying not to hit the string accidentally, just hit it purposefully — as I mentioned before when I was talking about something mentioned in the Josh Layne video about the Haendel harp concerto.
Starting in measure 9 of the exercise, you’ll also have to concentrate on placing 4 in the right hand firmly and well in order to avoid buzzing.
This is also helping toughen up my fourth finger, which is somewhat sore today from actually being used. 🙂