X-Gal staining of transfected cells: background

[P.C.]

Hi!

Sometimes when I do X-Gal staining of HeLa cells transfected 
with pCMVbeta I obtain blue cells in the mock sample. (this 
staining is not as strong as in the positive cells, but it 
makes a problem when transfection efficiencies are low)

Does any one know how to reduce this background staining ?
Thanks for any suggestions,

Peter.