actually brick walls are one of the easier things to make a seamless map of.  Its best if you start with a large image (many bricks) with little to no shadowing.

First step is the vertical tile ..generally easy if your bricks are uniform shapes. Just keep an eye if the bricks are coursed (rows are staggered).  

Then make a new layer copy of the texture and turn off its visibility. You'll want it in a minute.  Offset the original so the horizontal center is in the middle of the page. Now turn on your copy and erase everything but a culmn or two of bricks and a little mortar on each side. I use a slightly soft brush to avoid making hard edges.

then align your seam covering bit and smooth out the edges.

This same technique works on most brick, stone and tile textures. it gets trickier on fieldstones which are different shapes, or on moss covered walls.

Lyrra