Any tips for free mounting on a steep downhill? Happened to me today - I had to get off for a car going down an almost vertical hill, but couldn’t figure out how to adjust the mount (balance or rollback) to cope with the instant momentum I’d have. Walked down the rest of the way. 
If locals are reading its the western hills cemetery hill - the one closest to the sports park in Napier.
If you don’t feel up to static mounts on the downhill (you just need to apply more pressure on the rear pedal) you can mount fairly normally if you just turn to the side. 
+1 to getting comfortable with static mounts. Most useful and utilitarian mount there is. I’ve been riding for over 5 years, lots of long distance off road experience, trials, etc… but I couldn’t do a roll-back mount to save my life!
Your pedals should be flat relative to gravity, not to the hill. And the static mount will be easier than the roll-back.
Rollback mount would be terribly difficult on a steep downhill, but a static mount on a downhill is actually easier than on an uphill. Like Obtusellama said: simply apply more force on the rear pedal. If the slope is really steep you can (almost) stand on it and do the mount quite slow. Now what can be easier than that? In fact, most beginners find it easier to learn the static mount on a slight downslope than on the flat.
Is it the mount itself that you’re having trouble with, or the downward force that you have to cope while riding (i.e. immediately after the mount)? Either way, if you’re having trouble, work your way up (or down :)) from milder slopes.
Instead of pointing your wheel downhill, rotate 90 degrees. Once mounted ride or hop back to pointing downhill. This is handy for uphill as well.
i apologize for being a physics nerd, but you dont have any momentum unless you are moving. you would just accelerate faster if facing downhill.
but on the point, just do a static mount and put a lot of weight on the back pedal
You didn’t say what size of tire you’re riding.
My standard is the HOT mount. Hand On Tire. It’s the only way I can climb up on the Coker. It has become my standard muni also because it gives me more time to gently come down onto the seat for a couple of reasons.
+1
+1
It is a nice mount because I bring it to a still-stand on my Muni and then ride. On a down hill the HOT mount gives me a nice balanced position to start riding.
OK, perhaps I should have said potential momentum. I apologize for not knowing anything about physics.
Wish I did though!
I was riding my 29" (which I don’t have a good working relationship with) I can mount my 26" just about anywhere. I think it is all in my head to be honest.
HOT feels like cheating to me…
;)but I can see how a coker would need it (not that I have ridden a Coker, but I can imagine you’d need something like that.)