Tommy M:
Place the tip of your tongue on the roof of your mouth, just at the point where the gumline sinks down to the palate. There's a point slightly further back which is used in, I think, certain Taoist techniques, but it's not necessary for this purpose and not swallowing your tongue can become a distraction.
Tommy, personally do think that swallowing the tongue is a
valid concern?
I have never heard of it happening to anyone before, I can reach kechari mudra level 3 with a certain ease now, and never had to worry about "swallowing the tongue", BUT I never meditated for many consecutive hours, so I'm not sure, but I think the gag reflex would kick in anyway, so it wouldn't be a problem.