  We explore four different strategies to extract the D-meson semileptonic
decay form factors from the Green functions computed in QCD numerically on the
lattice. From our numerical tests we find that two such strategies, based on
the use of double ratios of 3-point correlation functions, lead to an
appreciable reduction of systematic uncertainties. This is an important step in
reducing the overall uncertainty in the lattice QCD results for the D-decay
form factors which are needed to determine the CKM entries |Vcd| and |Vcs|
experimentally, that are nowadays known by imposing the unitarity of the CKM
matrix.
