I have tried falling forward, and it definitely causes an energetic shift of some kind. It has either made my dream much more lucid, or I've woken myself up with massive energetic sensations tingling throughout me. And in one case I had a short astral trip to my kitchen in the RTZ.
Falling backwards has only caused my dream scenario to change.

Some people recommend jumping off a cliff.

I think that it's essentially the momentum that's important... causing enough of a feeling of "falling" that you trick your astral body into shifting out. I found that it helped if I fell forward with a lot of intention and really revved up my energy before I did it.