Here's what works for me, with the exception of a single, non-matching Bubba'd late Arsenal 26:
Position the safety as others have mentioned.
Make sure the magazine is fully "home".
Make sure the pins in the trigger group are fully "home".
Place the butt on a table or other suitable working surface (not the wife's glass-topped stove or porcelain-topped whatever), with the muzzle pointing straight up; I usually have the receiver to my left, and the trigger group to the right.
Place your left palm over the receiver, the center of your right palm over the trigger guard, and intertwine/interlock your fingers.
Now, compress the receiver and trigger mechanism together by pushing your palms together.
Like I said, I've only had this NOT work on a rifle that was an obvious assemblage of mismatched parts. I've never had to take a mallet to the first one; the difficult one only required a bit more pressure than hands could apply, and I did that by putting the receiver down on a wooden work bench, and then pressing down on the trigger guard with my right palm. But if you do this, make SURE that you've placed the takedown lever back in the the receiver and locked it into place; it will easily swing down when you least expect it against the table, and you may bend the @#$% out of it before you realize what's happened!