What I do is, when I’m learning a new kanji or vocab item I will say the readings (both on’yomi and kun’yomi in the case of kanji) out loud a few times, and I deliberately exaggerate the pronunciation those first few times. I let the こう stretch out nice and long, I cut off the こ sharply, and I make sure to make the difference the small っ makes sound through very clearly. And every time I fail a review because of those differences, I repeat the ritual.
(If I’m in company I won’t pronounce the readings out loud, but I still do them in my head and do the same exaggeration a few times.)
That way, the pronunciation sticks a lot better for me. Just make sure to do the pronunciation normally when you’re actually speaking Japanese
This works for me, your mileage may vary of course.