When shooting to prevent reflections, it's best if you are at an angle of around 35 degrees to the reflective surface.
Avoid using a polarising filter on a lens wider than 28mm as the effect can look false because only a proportion of the sky will be deeply polarised.
You don't have to have the polariser on the lens to see if it's going to be effective. Hold it up to your eye and rotate the ring. If it makes a difference to your scene, screw it on.
Watch the exposure. A polariser has a neutral grey look, which won't affect colour, but does reduce the amount of light reaching the sensor and you'll probably have to compensate for 1-2 stops.
Use two polarisers one over the light source behind a plastic subject and one on the camera for a cross polariser effect. There's a technique how to do that here: Cross Polarisation
Buy a slim version for use on a wide-angle lens to prevent the mount causing vignetting
How much you rotate the filter will change how effective it is. If you want to see fish below the surface of a pond, for example, you'll need to rotate it round to the strongest position. (In the above shots, the top image of the pond is taken without a polarising filter and the bottom shot, which shows the fish more clearly, was taken with a polarising filter on the lens.)
Don't use a polariser when shooting through an aircraft window as you will record distracting patterns in the window and ditch the polariser when working in low light or when you want to capture an image of a rainbow.
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