How to paint straight lines!

• using the RMB, angle the camera so you're looking straight at the wall you want to paint
  
• using the MMB, move the camera so the edge (probably the top edge, but whatever edge works for you) is lined up where you want your edge to be.
  
• bring the paintbrush all the way to the edge of the screen. you won't be able to move it past the screen.
  
• paint in a straight line against the edge of the screen, constantly trying to push the brush off the screen.
  
• the brush stays on the screen, up against the edge, resulting in straight painted lines

depending on how you angle your camera against the wall, you can angle your lines