Hi David,
I will share how I do things, but I am obviously no expert.
My second bag ended up with tons of knots showing, and it was embarrassing. I had a very generous young lady show me how to hide knots, and I think that through before any stitch goes in.
My dad used to made holsters, and in thick leather he used the same trick as the CapGunKid. In thin leather I have not had as much luck, and am scared of unravelling. I try to tie knots at the end of each run, and hammer them flat, and put a drop of superglue on each.
I try to leave a knot where it is hidden. For example, when sewing on the buckle, the knot is hidden between the layers of leather, up against the buckle. When you get to the last stitch, instead of going through both layers, angle your needles, go through only one layer so you end up with two tails hanging out from between the layers. Tie your knot, hammer it flat, and add a drop of glue.
If it is an inside out bag, I sew the body together using three runs. That way if one thread breaks, it only unravels part of the bag. I usually use heavy linen cord for this, so it is a stout seam. Hidden inside, no one will see knots.
On the bag you saw today, take the flap for instance. I started all stitching from the end towards the bottom of the flap. The knots were to the top, on the front face. I planned them there knowing the top hinge would cover them. That said, you have to play "chess" and think several moves ahead. The fanned out stitch rows do not go all the way to the top of the flap. That is to keep the horizontal stitching holding the flap to the hinge from cutting the fanned out lines. I thought that through before starting. That is where a well-thought-out pattern comes in handy.
The scary one on this bag was the line around the flap edge. It's a long run. I did it all with one piece of thread, to only have one knot to hide. Yes, it was a LONG piece of thread. I make my piece of thread four times as long as the run when saddle stitching, plus a little. My arms get tired before I am done, but there was no visible splice or knot when I was done.
There are masters like James Rogers, Lawrence Fiorillo and Greg Hudson who can share the correct way to do things. I am still on the learning program.
God Bless, Marc