  The application of dynamical decoupling pulses to a single qubit interacting
with a linear harmonic oscillator bath with $1/f$ spectral density is studied,
and compared to the Ohmic case. Decoupling pulses that are slower than the
fastest bath time-scale are shown to drastically reduce the decoherence rate in
the $1/f$ case. Contrary to conclusions drawn from previous studies, this shows
that dynamical decoupling pulses do not always have to be ultra-fast. Our
results explain a recent experiment in which dephasing due to $1/f$ charge
noise affecting a charge qubit in a small superconducting electrode was
successfully suppressed using spin-echo-type gate-voltage pulses.
