  Dirichlet boundary conditions on a surface can be imposed on a scalar field,
by coupling it quadratically to a $\delta$-like potential, the strength of
which tends to infinity. Neumann conditions, on the other hand, require the
introduction of an even more singular term, which renders the reflection and
transmission coefficients ill-defined because of UV divergences. We present a
possible procedure to tame those divergences, by introducing a minimum length
scale, related to the non-zero `width' of a {\em nonlocal} term. We then use
this setup to reach (either exact or imperfect) Neumann conditions, by taking
the appropriate limits. After defining meaningful reflection coefficients, we
calculate the Casimir energies for flat parallel mirrors, presenting also the
extension of the procedure to the case of arbitrary surfaces. Finally, we
discuss briefly how to generalize the worldline approach to the nonlocal case,
what is potentially useful in order to compute Casimir energies in theories
containing nonlocal potentials; in particular, those which we use to reproduce
Neumann boundary conditions.
