How to choose Gaussian surfaces while solving problems? I have a doubt regarding this problem:

Two large identical flat metal plates are placed parallel to one another, seperated by a small distance compared to their linear size. One plate is given a charge per unit area $-5.50 \times 10^{-5} Cm^{-2}$ and the other a charge per unit area $+5.50 \times 10^{-5} Cm^{-2}$. Using Gauss's law find the electric field at an arbritrary point between the two plates. 

I did it with the plug and chug formula without much trouble. However, when I am trying to use Gauss' Law, I don't know what surface to choose. I drew a cylinder but I'm not sure of its orientation (should I set the circular faces perpendicular or parallel to the field?). As well, I'm not sure on which surface to use when confronted with new problems. Is there a foolproof way of doing it or any set forms recommended? 
 A: First consider a problem with one metal plate.
Denote the charge density by $\sigma$. Since the problem has a rotational symmetry around an axis that is normal to the plates (plates are infinite), the electric field must be directed along such axis. Therefore, is makes sense to choose the gaussian surface to be a right cylinder with its axis normal to the plates. The base of the cylinder can have any shape (suppose round for simplicity) and has area $S$.
Now it is obvious that the flux of the electric field through the sides of the cylinder vanishes. On the two opposite bases of the cylinder, electric field $E_1$ is obviously directed in opposite directions (towards the plate if $\sigma<0$ and away from the plate otherwise). Therefore the total flux $\Phi_1$ in the case of a single plate is equal to
$$
\Phi_1=2E_1S.
$$
On the other hand, Gauss proved that (in SI)
$$
\Phi_1=Q/\varepsilon_0=\sigma S/\varepsilon_0. 
$$
It follows that $$E_1=\frac{\sigma}{2\varepsilon_0}.$$
We can now return to the initial problem with two plates. Between the two plates, the electric fields $E^+$ and $E^-$ produced by the plates (with charges $+\sigma$ and $-\sigma$ correspondingly) are equal as vectors, hence $$E=E^++E^-=\frac{\sigma}{\varepsilon_0}$$
between the plates. In the rest of the space, vectors $E^+$ and $E^-$ cancel out: $$E=E^++E^-=0.$$  
It is probably worth mentioning that there are very few problems, where there is a straightforward way of choosing a gaussian surface and using the flux theorem to find $E$. Generally you get an integral equation for $E$, which is equivalent to the Poisson's equation $\Delta\phi=-4\pi\rho$ (in Gaussian unit system) for the scalar potential $\phi$. Overall it is useful to choose the gaussian surfaces to be equipotential, so that at every point of the surface, $E$ is normal to the surface (though generally not constant along the surface). Sometimes some parts of the surface (like the cylinder's sides in our problem) can be chosen to be orthogonal to the equipotentials at every point, so that the flux through these parts is equal to 0. But such surfaces don't always exist.
