Gradually build up to a full pipe. From least amount of tobacco (first few smokes) to nearly full bowls. If you already have two or three pipes that are broken in, you can and should take your time. But don't worry too much about it, as the key thing is conditioning the walls to the heat of tobacco being burned. And in any case you'll ream the cake down a bit later on.
And note, by the way, that the greatest heat is generated in TWO areas: the top of the tobacco when you light it, and the bottom of the bowl when there's little left and what remains has been compacted by tamping. That last bit deserves careful attention, as it can start a burn-out in an as-yet unbroken-in pipe. Even with a very slow smoke, some of the juices are likely to be boiled into the wood also. Best not to obsessively smoke every last scrap in the pipe for a while, till the thing is well broken in at least. A few shreds remaining is quite okay.
One other thing to bear in mind is that tobaccos have different burning characteristics. Pure Virginia flakes are to be smoked slower than anything else, almost sipped. Wherefore they produce the least amount of heat. Burleys, too, can be slow and regular. English and Balkan are sometimes at their operatic best when hotboxed, and aromatics tend to be scorchers.
