Subbing! Caly lost a lot of her topline in the 100+ weather this summer and we're just now starting to build it back up and it's SLOW work. So we will be taking all this advice as well! Thanks for asking the good question, Army Wife!
I would say all of the things here are great suggestions.
And to your dying leg comment: I find the best way to get a horse into a nice, large, power walk is to "go with the rhythm of the horse." First get your horse walking and remain in a light, loose body position letting your hip and lower back sway with the horse. Then, when the right shoulder moves forward apply right leg to encourage that leg forward and take a larger, longer step. Then do the same with the left. Continue doing this and soon you will find your horse taking long, powerful strides using his hind end and back very efficiently.
This seems to be better because it is not as dull of an aid as constant leg pressure and this helps break it up to the horse, and it also helps the rider get "in tune" with the horse. You could also do this at the trot but this seems much more challenging...
I saw this at a clinic at the Equine Affaire but I forgot the clinician's name... I did it on my lesson horse once since he tends to be a slow, western pleasure type, and then we got him into a nice walk with a pretty good headset!
I have 4 horses and 3 of them had poor topline when we got them. But we live in a ravine and every time we ride out on the trail we have to go up a hill, so these poor horses have to climb hills daily. Plus the mud in our area is a thick clay, so they have to really pull their feet up. Now their top and backsides look amazing!
Someone on here also mentioned backing up the hill at a walk. We have been trying that, just 4 or 5 steps at a time.
And I always like the long long trot. That really builds a horse...and rider too!