Just like layer opacity in Photoshop, Lightroom ships with a handy Flow slider when you decide to locally adjust the photo using the Adjustment Brush. This is extremely handy if you want to apply changes in increments rather than full blown at 100%. Another handy option in the same dialog box is to select the A or B brushes to switch between states of the brush adjustments.
Let's put this to practice:
Let's say I want to add clarity to the left eye less than the right eye in a portrait. To do that just select B and lower the Flow slider to 20%, paint over the left eye. Then select A and increase the Flow to 100% and paint over the right eye. Lightroom will remember your A and B settings for next time.
Another bonus tip: if you hold down the letter "O" (for overlay) while painting, Lightroom will show you a red overlay on your image to indicate where you are painting. If you used a low Flow brush, this red will be lighter and for the right eye, this flow will be darker.