After many years I finally broke down and bought a balsa building board it was worth it as far as I am concerned.
Over the years I have tried many different materials to build models on and this is the best I have used thus far. I have tried pine and fir, cut and glued in butcher block fashion Homeasote and ceiling tiles and this works the best thus far. Pine board are never flat that I have seen it is the nature of the material to warp and bow, although I have seen some old growth pine that was amazing but you can not find that around much except in very old homes and antique colonial furniture.
Some guys swear by sheetrock but I would imagine that it has a finite lifespan, but it is cheap.


But the best system I have seen thus far is the magnetic building clamp systems such as described on http://www.airfieldmodels.com.