This is a link to a guys pretty good explanation of how to "lock to an object", trace and follow - with "not much brains", and just a single sensor.
Trick is, to put it short; Focus on the edge / start / end point of the object - not the object itself.
If you then focus on 2 sides while scanning, you can determine the middle of the object, and bang - you can have your robot follow another, look at your finger when moving it in front etc :)
