Perform the first digest in the low salt buffer. Then supplement with 100–150mM NaCl using a concentrated NaCl stock solution (i.e. 5 M NaCI). Alternatively, perform the low salt digest in a 20 µl reaction volume and then increase the reaction volume to 50 µl, by adding 5 µl 10X NEBuffer 3, BamHI, BSA and water.