Put a pipe's worth of tobacco on a paper plate and chop it into 3/16" (ish) lengths with a chef's knife, and pinch fill---meaning aspirin-tablet-sized pinches---one at a time followed by a bowl tap between each to distribute/settle it, until the chamber is full.
Test the draw and add another pinch at a time with a draw check between each one. When the draw is barely on the loose side of perfect, you're ready to go. (Usually takes three, sometimes four)
Light thoroughly, flatten the ash dome that pops up---NOT compress the tobacco beneath it---and light again.
I had a 1" diameter 2.75" deep bowl go to the bottom the other day without a single re-light. That's 2 hrs and 20 minutes.
The secret is creating a uniform 3D matrix of tobacco and air space. The "pinch at a time while tapping" thing is the only way I've found to reliably create it.
PS --- When off-loading ash during the smoke, just pour it out, and gently re-flatten. Do not knock on an ashtray cork or similar. The tobacco will expand from burning enough to keep it in place during normal pipe handling, but a whack CAN break it loose.