With the recently released v1.2.0 update for Voxel Paint, there are now some new keyboard shortcuts you should know of to help you with your voxel art. This thread will be updated as new shortcuts are added.

Shift
  • Pen and Eraser Tool: Holding shift while using the pen or eraser will activate free draw/erase mode.
  • Select Tool: When using the Select Tool, holding down on Shift before you click on a voxel will do a "Flood Select" operation and select all connecting voxels.
Ctrl + A: Select all voxels.

d: While in select mode, pressing the "d" key will duplicate the current selection, creating a copy.
g: While in select mode, pressing "g" will group the selected voxels. After this, you can click on one of the voxels in the group to select that group again.
u: Ungroups the selected voxels.

r: When you are using the Move Tool or pasting duplicated voxels, pressing "r" will rotate the voxels you are moving by 90 degrees.

Escape: The escape key will deselect all voxels and also cancel a pending voxel duplication operation.