First, watch your bluebird houses.  If you see sparrows entering and exiting the house, keep tearing out their nests.  Eventually, they will leave and find another home. 

You can attract bluebirds by offering mealworms, which are available fresh, freeze-dried, and mixed in with other seeds.  You’ll want a feeder which will not allow larger birds to feast on your offering, like the Looker Products Bluebird/Chickadee feeder.

Another house, which bluebirds will use, but sparrows generally won’t is a slot style bluebird house, also offered at Barn in the Sticks.  Also, ensure that the entrance to the house meets the specifications for the bluebird you are trying to attract.  The mountain and eastern bluebirds will use a 1-1/2” hole, while the western bluebird prefers a 1-9/16” entrance.  Anything larger than 1-9/16" could encourage starlings to your house.