First of all, find plans so you don't get bunkers looking like many of those made by Verlinden Productions
As for the actual construction, Jordan's method probably gives the best results, but is also the most time-consuming. Another method is to use clay, which you can roll out into sheets (use two bits of wood to make sure it's of an even thickness) and then pressing the imprints of the planks, used for the mold on the real thing, into the clay with a strip of wood. I'll plug my bunker pics site, which should have some good views for you to see what the texture on a cast bunker wall looks like: http://www.xs4all.nl/~gurth/afv/bunkers
especially the type 501 and 502 pages.
An alternative, if you want to make a German bunker anyway, is to build one that wasn't cast into a wooden mold, but into one made of Formsteine
mold bricks. These were fairly large, concrete bricks of which an inner and outer wall were constructed around the steel reinforcements, with concrete then being poured between them. The bricks became part of the bunker wall, and give it a very recognizable, square appearance. The great advantage of this type of construction for modellers is that you can make the bunker from flat panels and scribe the bricks into them, without the need to reproduce the marks left by the wooden molds more commonly used. However, this method wasn't used often, so you may want to check if it was in the area your bunker is supposed to be in.