  We use a recent numerical security proof technique to analyse three different
postselection strategies for continuous-variable quantum key distribution
protocols (CV-QKD) with quadrature phase-shift keying modulation. For all
postselection strategies studied we provide novel analytical results for the
operators that define the respective regions in phase space. In both, the
untrusted and trusted detector model, a new, \emph{cross-shaped} postselection
strategy, clearly outperforms a state-of-the-art radial postselection scheme
for higher transmission distances and higher values of noise. Motivated by the
high computational effort for the error-correction phase we also studied the
case when a large fraction of the raw key is eliminated by postselection: We
observe that the secure key rate in case that only $20\%$ of the raw key passes
the cross-shaped postselection is still roughly $80\%$ of the secure key rate
without postselection for low values of excess noise and roughly $95\%$ for
higher values of excess noise. Furthermore, we examine a strategy with radial
and angular postselection that combines both the advantages of state-of-the-art
radial postselection schemes and our cross-shaped strategy at the cost of
higher complexity. Employing the cross-shaped postselection strategy, that can
easily be introduced in the data processing, both new and existing CV-QKD
systems can improve the achievable secure key rates significantly.
