Ctrl+shift+click to toggle selection of a face. You can also use MMB to copy a texture from a surface and then ctrl+shift+MMB to paste.
You can also use ctrl+c to copy the texture from the selected face, and ctrl+v to paste it somewhere else.
Cheap trick: if you need to paint several faces with the same texture: select all the brush faces using
ctrl+shift+click, and last select a face that already has the desired texture.
First press ctrl+c, followed by ctrl+v. All selected faces will get the same texture as the face you last selected.