  We propose a method for reducing the sensitivity of atomic ground to Rydberg
transitions to stray dc electric fields, using microwave-induced dressing of
Rydberg states. Calculations are presented for the Cs $\state{90S}{1/2}$ and
$\state{90P}{3/2}$ states. With zero dc bias electric field, a two-frequency ac
field is used to simultaneously reduce the sensitivity of both states to dc
field variations. The sensitivity reduction is a factor of 95 for the
$\state{90S}{1/2}$ state and a factor of 1600 for the $\state{90P}{3/2},
m_J=3/2$ state.
  We also show how the two-frequency ac field can be used to cancel both
second- and fourth-order terms in the polarizability of a single Rydberg state.
These results are relevant to improving the stability of experiments that seek
to excite Rydberg atoms in the proximity of charged surfaces.
