I would use `the following equation` in a sentence immediately preceding the equation. I would use `the equation below` if there were more text in between. 

I would tend to avoid these formulation to be frank, I would re-organise the text to avoid forward references to an equation in general, unless I speak about a celebrated equation. For example `The law or large numbers below has first appeared in... `, `Einstein's equations below describe...`. Compare with `The following equation is the strong law of large numbers: [Displayed Eq.]`.

If I refer to an equation above, I number it and refer to the number: `In Eq. (xx), $a$ is the acceleration, ...`