Adjusting the sun location

To adjust the sun location in order to view the terrain in different lighting conditions, hold the 'alt' key down, and drag the mouse with the right button held down.

    Auto create land and water

Press 'Ctrl-F' to make high areas land and low areas water.

    Smoothing Shorlines

After painting in oceans, rivers, or other features that may result in rough shorelines, hit 'Ctrl-S' (or the smooth button) to smooth out the shorelines.  You may select an area to smooth only the shorelines in that area. 
Hint: To quickly select a feature such as a river, simple double click it in selection mode

    Thinning areas

The thin tool is very handy for creating features such as narrow rivers, or pathways.  To use it, pain a river or pathway, then select it (by double clicking it in face mode) and then use the thin tool (Shortcut is Ctrl-T by default).