I have the same issue sometimes. Not sure if this will work for you too, but if I can remember to do this in the actual event of me getting caught on an m at the beginning of the word, this helps me: I drag out the m into a humming sound, so it's not "milk" but "hmmmmilk". Try to make your lips touch as little as possible without actually opening your mouth and don't think of saying the word, but just the hum. As soon as I focus on the word, I close off and can't get the word out, but if I hum first and then think of the word again while humming, it works out fine. It took me some time to practice this enough so that I don't just stand there humming for three seconds, but now it just sounds normal. Another thing that helps (especially with short words like milk) is breathing out when saying the rest of the word after the m. Releases some of the stress. Hope this helps.