My stutter is more of a block stutter and get stuck on those letters somewhat often. I have noticed during my hard blocks on those sounds it's usually when I am pressing my lips together really hard, my larynx is closed hard, and I have a lot of pressure built up in my mouth. I have practiced not allow those three things to happen. I'll make to relax my throat, not press my lips firmly, and not let pressure build up. It seems to help when I remember it. I don't always remember when in speaking situations though.