  We analyze and compare three different strategies, all aimed at controlling
and eventually halting decoherence. The first strategy hinges upon the quantum
Zeno effect, the second makes use of frequent unitary interruptions
("bang-bang" pulses and their generalization, quantum dynamical decoupling),
and the third of a strong, continuous coupling. Decoherence is shown to be
suppressed only if the frequency N of the measurements/pulses is large enough
or if the coupling K is sufficiently strong. Otherwise, if N or K are large,
but not extremely large, all these control procedures accelerate decoherence.
We investigate the problem in a general setting and then consider some
practical examples, relevant for quantum computation.
