  We show that, above the critical temperature, if the dimension D of a given
Ising spin glass model is sufficiently high, its free energy can be effectively
expressed through the free energy of a related Ising model. When, in a large
sense, D is infinite, in the paramagnetic phase and on its boundary the mapping
is exact. In this limit the method provides a general and simple rule to obtain
exactly the upper phase boundaries. We provide even simple effective rules to
find crossover surfaces and correlation functions. We apply the mapping to
several spin glass models.
