The paper studies the structure of high-order adiabatic approximation of a
wave function for slowly changing Hamiltonians. A constructive technique for
explicit separation of fast and slow components of the wave function is
developed. The fast components are determined by dynamic phases, while the slow
components are given by asymptotic series evaluated by means of an explicit
recurrent procedure. It is shown that the slow components represent
quasiadiabatic states, which play conceptually the same role as energy levels
in stationary systems or Floquet states in time-periodic systems. As an
application, we derive high-order asymptotic expressions for quasienergies of
periodic Hamiltonians. As examples, a two-state (spin-1/2) system in
periodically changing magnetic filed, and a particle in moving square potential
well are studied.