Best posture to ride a road-bike down hill?
This answer will be for riding down hills fast, but a lot of the advice applies to slower speed descents.

When riding downhill you should have your hands in the drops, the pedals at level (i.e., one foot forward, one back) and depending on your flexibility you want to get your butt back and bring your head down close to the stem so you don't present your chest to the wind. You also want to avoid a "death grip" as this can make the bike unstable.

This position isn't entirely about aero (for that you can do the death tucks like lying on the top tub your getting your head and upper shoulder ahead of the handle bars forming a wedge). You want a weight distribution that makes the bike stable, but responsive enough that you can avoid obstacles and navigate turns.  Getting into the drops shifts weight forward which slows down the responsiveness of the steering to light input but lets you carve harder turns.  Conversely if you put all your weight on the back the steering becomes lighter which can be unstable at higher speeds.  The drops is also the strongest position on the bike in terms of physical leverage.

I also like to hover above the saddle, that way if I hit a pot hole or dip the bike doesn't become unsettled.