  We derive an improved prescription for the merging of matrix elements with
parton showers, extending the CKKW approach. A flavour-dependent phase space
separation criterion is proposed. We show that this new method preserves the
logarithmic accuracy of the shower, and that the original proposal can be
derived from it. One of the main requirements for the method is a truncated
shower algorithm. We outline the corresponding Monte Carlo procedures and apply
the new prescription to QCD jet production in e+e- collisions and Drell-Yan
lepton pair production. Explicit colour information from matrix elements
obtained through colour sampling is incorporated in the merging and the
influence of different prescriptions to assign colours in the large N_C limit
is studied. We assess the systematic uncertainties of the new method.
