  We report on spin coherence lifetime extension on Tm$^{3+}$:YAG obtained
through dynamically decoupling the thulium spins from their magnetic
environment. The coherence lifetime reached with a Carr-Purcell-Meiboom-Gill
sequence revealed a 450-fold extension [$\sim$$(230 \pm 30)$ ms] with respect
to previously measured values. Comparison to a simple theoretical model allowed
us to estimate the correlation time of the fluctuations of the ground level
transition frequency to $(172 \pm 30)$ $\mu$s at 1.7 K. For attaining efficient
decoupling sequences, we developed a strategy inspired by the
\emph{zero-first-order Zeeman} effect to minimize the large inhomogeneous
broadening of the ground level spin transition.
