Oh also, if your texture map is super-warpy (like the top of the cube) it can be tricky to paint.
You can use the "bake texture" plugin to work-around that. Duplicate your shape, re-map it with some sane uvs, and paint it as desired. When you are done, bake the texture from the sanely-mapped shape back to the sculpted shape. You'll have a much easier time dealing with difficult texture coordinates that way.
http://www.inivis.com/forum/showthre...t=bake+texture