  We give a new, two-step approach to prove existence of finite invariant
measures for a given Markovian semigroup. First, we identify a convenient
auxiliary measure and then we prove conditions equivalent to the existence of
an invariant finite measure which is absolutely continuous with respect to it.
As applications, we give a short proof for the result of Lasota and Szarek on
invariant measures and we obtain a unifying generalization of different
versions for Harris ergodic theorem which provides an answer to an open
question of Tweedie. We show that for a nonlinear SPDE on a Gelfand triple, the
strict coercivity condition is sufficient to guarantee the existence of a
unique invariant probability measure for the associated semigroup, once it
satisfies a Harnack type inequality. A corollary of the main result shows that
any uniformly bounded semigroup on $L^p$ possesses an invariant measures and we
give some applications to sectorial perturbations of Dirichlet forms.
