This is why I like the Mika Pocket Holster.
It stays in the pocket, as it should, and it doesn't require a push of the thumb, which can be forgotten in the heat of a deadly encounter. The only downside to the Mika is, while it hides the gun well, it doesn't reatain the gun well when you stand on your head.
Last night I was lying down on the couch with my BUG in it's respective Mika Pocket Holster. Through my various movements the little revolver I carry ended up out of my pocket and on the floor. That is the only drawback to the Mika, and one I'm willing to live with since it's perfect in every other consideration IMO.
I've used Pocket Holsters with a "grab hook" on them that catch the holster on the edge of the pocket. These don't work too well for me, since I have managed to draw the gun and holster at the same time before. Everything has a trade-off and you will have to decide what works for you. Since I don't spend much time lying down on my back, unless I'm at home in bed or on the couch, I'm willing to face the possibility of a gun falling out of my pocket should I end up in that position.
Biker