  We study decompositions of operator measures and more general sesquilinear
form measures $E$ into linear combinations of positive parts, and their
diagonal vector expansions. The underlying philosophy is to represent $E$ as a
trace class valued measure of bounded variation on a new Hilbert space related
to $E$. The choice of the auxiliary Hilbert space fixes a unique decomposition
with certain properties, but this choice itself is not canonical. We present
relations to Naimark type dilations and direct integrals.
