  We demonstrate significant improvements of the spin coherence time of a dense
ensemble of nitrogen-vacancy (NV) centers in diamond through optimized
dynamical decoupling (DD). Cooling the sample down to $77$ K suppresses
longitudinal spin relaxation $T_1$ effects and DD microwave pulses are used to
increase the transverse coherence time $T_2$ from $\sim 0.7$ ms up to $\sim 30$
ms. We extend previous work of single-axis (CPMG) DD towards the preservation
of arbitrary spin states. Following a theoretical and experimental
characterization of pulse and detuning errors, we compare the performance of
various DD protocols. We identify that the optimal control scheme for
preserving an arbitrary spin state is a recursive protocol, the concatenated
version of the XY8 pulse sequence. The improved spin coherence might have an
immediate impact on improvements of the sensitivities of AC magnetometry.
Moreover, the protocol can be used on denser diamond samples to increase
coherence times up to NV-NV interaction time scales, a major step towards the
creation of quantum collective NV spin states.
