  We discuss the details of calculating hadron properties from the OPE for
correlators of quark currents in QCD, which constitutes the basis of the method
of QCD sum rules. The main emphasis is laid on gaining control over the
systematic uncertainties of the hadron parameters obtained within this method.
We start with examples from quantum mechanics, where bound-state properties may
be calculated independently in two ways: exactly, by solving the Schroedinger
equation, and approximately, by the method of sum rules. Knowing the exact
solution allows us to control each step of the sum-rule extraction procedure.
On the basis of this analysis, we formulate several improvements of the method
of sum rules. We then apply these modifications to the analysis of the decay
constants of heavy mesons.
