Dynamic Brush Tool

tooldyna

Add custom smoothing dynamics to your brush. This will give you similar smoothing results as the normal freehand brush. There are a couple options that you can change.

Mass
Average your movement to make it appear smoother. Higher values will make your brush move slower.
Drag
A rubberband effect that will help your lines come back to your cursor. lower values will make the effect more extreme.

Recommended values are around 0.02 Mass and 0.92 Drag.