So what's the consensus on pushing up from the rails vs. pushing up from the deck?
There's no doubt my pop-up is a problem. The best surfing I've done in my life has been on waves so long that I could, when necessary, recover my rhythm after correcting the kinks from a lackluster pop-up--correcting them over one or two sections, then pulling it together for the multiple ensuing sections. (Not an available crutch 'round here, obviously.)
Many wise folks have insisted that 80% of surfing, if not more, is the take off. It sets the tone for all that follows.
Me, I definitely have that old man hunched-over pop-up hang-up. (Fatigue exasperates that bad habit, but I think it's always there to some degree.) Sometimes, I wish I could wear a brace that would keep my spine straight...
Any good advice for dealing with--or training to fix--that bad habit?
I've DEF got advice - I think I've studied this to the point of exhaustion LOL - and perhaps finally, I feel like I'm making some decent headway and I can feel it in my own surfing - which has progressed (in my own opinion) quite nicely over the past handful of months from "pretty consistently bad" to "not always totally incompetent"
But just like a physician doesnt need to have diabetes to know how to treat it, my own pop-up doesnt need to be perfect to help others perfect theirs.
So - specific to you and the description you provided, think about this:
1. All hands on DECK. Fingers around the rails cause drag in the water which slows you down. You may think it's imperceptible, but anything that causes drag is not good. Heck - it's why the pros so often surf leashless (or use "comp-light" leashes), they don't even want the drag of a leash. Fingers around the rails also puts you that fraction of an inch lower in the water, also not desirable.
2. Slide your hands further back towards your waist - if forces your upper body UP and helps minimize or even eliminate the "old man hunch"
3. Whenever you can and the wave/situation allows (which should be most of the time), ARCH YOUR BACK and get your head up high as you slide your hands back into position for the pop-up, BEFORE you actually pop up. Getting your back arched lifts your chest which makes the popup lighter and faster. Getting your head up high gives you a WAY better view of the wave and how you want to set up your ride - leading to a far better ride.
All that said, you surf better than me, dude - so take it all for what it's worth - but I hope it helps!!