First go to User Preferences->Addons and enable the EdgeGrow addon
in the Mesh category.
Go to EditMode, select one edge or connected edge-snakes.
Invoke it (button in the Mesh Tools panel).
Use the left and right arrow keys to grow / ungrow the edgeloop in any
direction.
The addon will show the edges you can grow next in light blue, with the
active edge(will be selected
when using arrow keys) highlighted in red. Use up and down arrow keys
to activate the other light blue
edges, this way you can route your edge-snake in every possible direction;
default edge is the middle one.
You can grow multiple slices at the same time.
If you wish to hotkey EdgeGrow:
In the Input section of User Preferences at the bottom of the 3D View >
Mesh section click 'Add New' button.
In the Operator Identifier box put 'mesh.edgegrow'.
Assign a hotkey.
Save as Default (Optional).