The easiest way I have found to make realistic brick and block walls is go to a hobby shop that carries Model Rail Road supplies and look in their Plastruct section. Find something that will work for you and buy one bag of it.
Next go to a store that sells the THICK Plastic No Parking, Keep Out, For Sale, exc. signs and buy a couple of them. They are much cheaper than the blank Plastruct stock and work just as well, if not better. Check out my Oasis Convenience Store thread to see what I'm referring to if you need to. (Point of interest - Buying the signs that are printed on one side only makes laying out designs much easier, just lay everything out on the unprinted side)
For the next part you will need a METAL ruler that has a millimeter scale on it, a SHARP #2 Pencil, and a good hobby knife with a fresh blade in it preferably. Unless you live alone or have a dedicated building area, a good cutting mat is recommended to keep you from getting yelled at for scratching up the table. Trust me, sanding and refinishing a kitchen table so you can quit sleeping on the couch is no fun!
Get out the Plastruct panel you bought for reference and measure the bricks or blocks, depending on what you are trying to replicate. Once you have those measurements, transfer them to the plastic sign, use the ruler for a straight edge (thus the need for a metal ruler - they are usually true on the edges), and using the back of your hobby knife scribe in the lines making sure that you check to keep from going completely through and that it's close to the depth of your reference material.
Pointers (In no particular order):
1. Doing all the long horizontal sections first and the short vertical sections last has been easier for me than working on one section and moving to the next..
2. Use light pressure and make several passes to reach the desired depth results in a cleaner, more believable panel.
3. Cutting the plastic sign to the desired shape and size FIRST is easier than trying to work with a panel that has been weakened with the scribed lines in it.
4. Blood does NOT work well as a lubricant! Be careful when scribing the lines, remember that the sharp side is now on the outside.
5. Plastic signs are CHEAP, so get an extra one to practice on before moving to the finished project.
6. If the printed side of the sign is going to be seen, sand the writing off with a fine grade sand paper BEFORE you do any work with the sign. Sanding a large area is MUCH EASIER than a whole bunch of little ones.
And there you have it, a cheap way to replicate brick and block walls that after you paint them no one will be able to guess what they were originally made from. On the plus side, you now have the original (and expensive in my opinion) Plastruct piece to use as a reference for later builds if so desired.
I hope this gives you some creative ideas for your project,
-Scott H.
P.S.
They also make great concrete floor panels if you scribe the joint lines in them.