What works for me, I'm sure there are dozens of ways to fix it. I take the sight out, place it upside down over a depression lengthwise, then give it a whack with a punch to give the base a SLIGHT concave surface. Should slide back in with a lot more friction now. You can also take a metal punch and swage the dovetail slightly tighter. This must be done in a careful manner so your cleanup on the barrel is minimal.
-Eric