When one is about halfway through a bowl of tobacco there is usually a good bed of ashes on top of the burning embers. One popular method of building cake is to spread some of the excess ash over the inside of the tobacco chamber. This is known as "Ashing".
The way this is done is to place one hand over the bowl of the pipe then gently shake the pipe and allow the fine ash to coat the exposed wood of the tobacco chamber. After doing this you can dump the remaining excess ash then gently tamp and continue smoking the second half of the bowl. Once the bowl is finished, repeat the ashing process one more time then dump the remaining ash. What this does is leave a good coating of tobacco ash along the interior of the bowl. This promotes the building of a good even carbon cake. Some folks will wipe out the shank with a pipe cleaner then smoke the pipe again (once cooled down) with the ash still coating the bowl walls. Doing this helps to "fuse" the ash and tobacco to the wood and therefore provides an excellent base for a good healthy cake.
Once you have built up a respectable cake, proper maintenance is necessasry to avoid the problems that can come from having too much cake. If a pipe has too much cake it can make the bowl heat up to the point where the bowl can crack open and end up as firewood. To avoid this you need to periodically ream the cake. This can be done with sandpaper or a pipe reamer.
The proper thickness for cake is equal to the thickness of a dime. When you cut the cake you should slowly and gently ream the bowl and remove the cake evenly and without creating a crack in the cake. Doing so will cause the cake to split and fall off in a large chunk leaving some exposed wood and an uneven cake. Dump the excess carbon powder from the bowl then whisk out the bowl with a bent pipe cleaner. Check the cake for proper thickness and uniform surface. Continue reaming as necessary.
With proper care and maintenance, you should have a pipe that smokes well and will last for many years.