Select the edges, then extrude them out using the Blue Handle on the default manipulator that appears when you press extrude. (See Image #1)

This should leave the edges relatively straight but not 100% straight. A Quick way to fix this is to select the Vertices's of the edgeloop extruded, and the edges that They originated from (the edges you selected when you performed the extrusion) And simply scale them down until it's flat.
You will have to do this to the bottom and side edges separately. (see Image #2)