  We study the optimization of nonperturbative renormalization group equations
truncated both in fields and derivatives. On the example of the Ising model in
three dimensions, we show that the Principle of Minimal Sensitivity can be
unambiguously implemented at order $\partial^2$ of the derivative expansion.
This approach allows us to select optimized cut-off functions and to improve
the accuracy of the critical exponents $\nu$ and $\eta$. The convergence of the
field expansion is also analyzed. We show in particular that its optimization
does not coincide with optimization of the accuracy of the critical exponents.
