We present a new approach to determine an accurate variational wave function for general quantum spin models, completely defined by a consistency requirement with the simple and well known linear spin·wave expansion. With this wave function, it is also possible to obtain the correct behavior of the long distance correlation functions for the 1D S=1/2 antiferromagnet. In 2D the proposed spin· wave wave function represents an excellent approximation to the exact ground state of the S=1/2 XY model. We obtained accurate values for the correlation functions and discuss their physical relevance. Since the discovery of High- Tc superconductivity increasing attention has been given to the study of strongly correlated systems. In particular, the role of antifer romagnetic (AF) correlations in such electronic systems has become clear, as they may lead to superconductivity at finite doping. l ) Since the main physical processes in High- Tc superconductors occurs in the copper-oxide planes, the role played by two spatial dimensional structures is of particular interest at the moment. Unfortunately, in two dimensions (2D) there is no exact solution, not even for the undoped AF Heisenberg model, and one must rely on approximate results or numerical calcula tions. Spin-wave theory is probably one of the most successful approximations in the theory of strongly correlated electrons. It is based on an expansion in the parameter l/S around the classical large spin solution, where S is the magnitude of the interact ing spins. The parameter l/S is quite large in the physically interesting case, S=1/2. Nevertheless, it has recently been shown that the spin-wave theory is very accurate in two and higher dimensions, yielding an extremely accurate determination of the order parameter and the energy per site when using only a few terms of the expansion. 8 ),6),7) Unfortunately, the spin wave theory represents a rather abstract mathematical tool as the physical Hilbert space for S=1/2 is obviously not conserved by the l/S expansion, and it is difficult to understand the meaning of the quantum corrections to the classical large spin solution. For the above reasons it is clearly useful to determine a wave function, defined for any value of the spin S in the physical Hilbert space, which is consistent with the spin-wave expansion for large S and that remains quite accurate for S=1/2. We will show here that this project can be easily carried out, allowing for the determination of an almost exact wave function for the 2D XY model, a model which is particularly interesting for its possible relevance in strongly correlated systems. The method is carried out on a finite lattice cluster, where a direct comparison