Lightroom CC / 6: Shift-Click to Constrain Brush
Lightroom CC 2015 / Lightroom 6 now allows you to constrain your brush movement. You can click to set a start point, then shift-click to set an end point, and LR will draw a perfect line between the two points. Alternatively, you can hold shift while painting, and Lightroom will constrain the movement only to horizontal or vertical lines.