Traditionally, I've used that little box knot that Robby posted, (the pictures with the leather fall, variations on both the image marked "1" and "2",) but after changing a popper on a whip that I'd loaned out to a student many moons ago, (fall was thick white hide and hadn't been cracked in a while, so while conditioned, the fall was a little stiff.) I've started using that in-line "tie the knots in the string, not the leather" method you can see in the 3b pic Robby posted.
For the most part, I really like it, and have had VERY good luck keeping my poppers in place.
Cracks seem just a little sharper and crisper with that whip now. I think the whip is cracking a little more efficiently, as tying a knot in the FALL makes this little ball of concentrated mass at the end of the fall, creating a minor interruption in the taper. That last knot for the most part just carries out that continuous line and continuous taper to the fluffy end of the popper.
I've since reset a popper on another whip using this method, (reset, not replaced. The popper came off, and I reattached it differently,) and have noticed a slightly improved volume and ease of cracking there too.
Really though, the differences are minor, but I DO think I'm putting a bit less stress on the leather of my falls by not putting small, tight knots in them that break down the leather over time.