An instructer here pasted the below links in another thread. Check them out.
The idea of practicing a scale on two strings is contained & expanded upon in these tutorials.
http://www.guitartricks.com/tutorial.php?input=928
http://www.guitartricks.com/tutorial.php?input=929
Then you can expand the idea across the fretboard!
http://www.guitartricks.com/tutorial.php?input=419
http://www.guitartricks.com/tutorial.php?input=898
http://www.guitartricks.com/tutorial.php?input=899
http://www.guitartricks.com/tutorial.php?input=453
http://www.guitartricks.com/tutorial.php?input=887