Welcome to the first installment of The Rigorous Rigger, where our motto is "I may not sail too good, but I rig REAL good!"
My ability to downhaul a sail has improved dramatically in the last few years, despite my getting older and tireder, due to two changes I've made in how I do it.
BIG TRICK #1: Rig like Robbie rigs, and use the newfangled downhaul threading pattern that manufacturers recommend.
It's astonishing to me how much easier downhauling is with this threading pattern, compared to what I'll call the "spiral" where you just make sure that the lines never cross over one another. A side benefit has been that since it's less backbreaking to downhaul, it encourages me to make more subtle adjustments to the downhaul, helping me get the sail tuned real good. So: Less friction, easier to downhaul, easier to adjust, and once you use this pattern a few times, it's easy to remember. (I learned this from the pamphlet that came with one of my Naish sails which is where "fig. 1" came from.)
BIG TRICK #2: Switch to Spectra or Formuline for your downhaul line. Back in 2005 I was in the Avon Sail Shop in Cape Hatteras, listening to Margaret explain why my friends and I should spend twice as much money on our downhaul line (spectra was stronger, it would last longer etc.) I was unmoved by the pitch, but I liked the shop so much I bought two for the heck of it. A month later I needed to replace a downhaul, did so with my new pricey Spectra, rigged up and noticed immediately how much more easily the sail downhauled. A sail that I used to have to finish downhauling with a crank now was no problem to downhaul solely with a cleat! Margaret didn't say "THEY'RE WAY EASIER TO DOWNHAUL" but they sure are. If you ever thought of putting a lubricant on your line (which is probably a bad idea) don't bother...these lines slide as if they're pre-lubed.
It's still worth noting that a downhaul crank will save your back and mine from stress and potential injury. I just find the above to make the entire process easier and more enjoyable.
(Thank you Dee Imbert, star of Ghost Stories, The Search for One-Eyed Jimmy, and the windsurfing cult microclassic The Ginsu Fin for naming The Rigorous Rigger.)