I'm not very familiar with the physics behavior myself, but I assume to have constant motion you would need to apply force equal and opposite of the linear or angle dampening. I have no idea how the dampening value is calculated. Note this won't actually get your object moving, it will only sustain movement. To get the object moving in the first place, you will need to apply a force or impulse greater than the amount dampened.
On the other hand, if your situation allows for it, you can simply set your dampening to 0.