It is well known that the dynamics of a Hamiltonian system depends crucially on whether or not it possesses nonlinear resonances. In the generic case, the set of nonlinear resonances consists of independent clusters of resonantly interacting modes, described by a few low-dimensional dynamical systems. We show that 1) most frequently met clusters are described by integrable dynamical systems, and 2) construction of clusters can be used as the base for the Clipping method, substantially more effective for these systems than the Galerkin method. The results can be used directly for system with cubic Hamiltonian. PACS numbers: 47.10.Df, 47.10.Fg, 02.70.Dh 1. Introduction. A notion of resonance runs through all our life. Without resonance we wouldn't have radio, television, music, etc. The general properties of linear resonances are quite well-known; their nonlinear counterpart is substantially less studied though interest in understanding nonlinear resonances is enormous. Famous experiments of Tesla show how disastrous resonances can be: he studied experimentally vibrations of an iron column which ran downward into the foundation of the building, and caused sort of a small earthquake in Manhattan, with smashed windows and swayed buildings [1]. Another example is Tacoma Narrows Bridge which tore itself apart and collapsed (in 1940) under a wind of only 42 mph, though designed for winds of 120 mph. Nonlinear resonances are ubiquitous in physics. Euler equations, regarded with various boundary conditions and specific values of some parameters, describe an enormous number of nonlinear dispersive wave systems (capillary waves, surface water waves, atmospheric planetary waves, drift waves in plasma, etc.) all possessing nonlinear resonances [2]. Nonlinear resonances appear in a great amount of typical mechanical systems such as an infinite straight bar, a circular ring, and a flat plate [3].The so-called "nonlinear resonance jump", important for the analysis of a turbine governor positioning system of hydroelectric power plants, can cause severe damage to the mechanical, hydraulic and electrical systems [4]. It was recently established that nonlinear resonance is the dominant mechanism behind outer ionization and energy absorption in near infrared laser-driven rare-gas or metal clusters [5]. The characteristic resonant frequencies observed in accretion disks allow astronomers to determine whether the object is a black hole, a neutron star, or a quark star [6]. Thermally induced variations of the helium dielectric permittivity in superconductors are due to microwave nonlinear resonances [7]. Temporal processing in the central auditory nervous system analyzes sounds using networks of nonlinear neural resonators [8]. The non-linear resonant response of biological tissue to the action of an electromagnetic field is used to investigate cases of suspected disease or cancer [9].