Prevent digging from becoming a problem: 

Give your dog the opportunity to dig in an appropriate location.  Teach the dog to dig only there.  Make it a really exciting "digging pit" – load it with buried toys, freeze-dried-liver, bones etc..  Go outside with your dog, supervise.  If the dog starts to dig elsewhere, correct with a sharp verbal reprimand, "Rover, dig in your pit".  Run over to digging pit, entice dog to dig, and praise. 

Entrapment: 

Place 1 to 2 drops of chicken broth or bacon drippings on the lawn.  Do some gardening/suntanning, but WATCH your dog.  If the dog makes one scratch at the ground, shout "Rover, dig in your pit".  Run to digging pit, entice dog to dig, praise.  Then leave dog in backyard and watch from a window.  As soon as the dog makes a move to scratch, tap on the window, then run outside shouting your instructive reprimand.  Entice dog to dig in pit, praise.