Prepare your rehearsals with percussionists in mind. Can the person playing bells play when you're hearing the flutes. Include the BD/SD when you're hearing the tubas on the march.
Avoid saying things like "listen to the band in front of you" and other similar small sayings we might use that subtlety imply percussion is not part of the band.
Give percussionists music when you give wind players music.
Include them in your warmups/daily drill. You might have to write something out for them.
Make it a goal provide percussion feedback at least once a rehearsal (very modest goal).