Decoherence is among the biggest obstacles for implementing high-performance quantum computing. Possible measures for reducing decoherence include dynamic decoupling (DD), i.e. sequences of inversion pulses applied to the system to be protected. These pulses effectively refocus the interaction between system and environment that drives the coherence decay. For a static environment, they can completely refocus the effect of the environment, resulting in long-lived coherence. We have explored this approach experimentally, using 13C nuclear spins (I=1/2) as the system qubit and a system of coupled 1H nuclear spins as the environment. Our results show that it is possible to extend the coherence time of the system by several orders of magnitude with the help of dynamic decoupling. Care must be taken that the unavoidable imperfections of experimentally realisable rotations do not accumulate throughout the sequence. We therefore designed and tested sequences that compensate the imperfections of the individual pulses over one cycle. The resulting sequences were shown to be very robust and can extend the coherence time of the system by orders of magnitude, independent of the initial state of the system.