  We give a progress report on studies of B_s and B_d mixing with valence NRQCD
b quarks and asqtad light quarks on the MILC configurations including the
effect of 2+1 flavours of sea quarks. We explore methods for reducing
statistical and systematic errors in the ratio \xi =
f_{B_s}\sqrt{B_{B_s}}/f_{B_d}\sqrt{B_{B_d}}.
