Elbow grease. Get a non-scratch kitchen scourer (I banish the old ones from my kitchen to the bathroom or laundry after a soak in bleach), and even some dish liquid (in a bucket, foamed up with some warm water). Dish washing liquid isn't usually as harsh on skin (or breathing it in!) as shower cleaners, and just give it a real good go. Just do a bit at a time if that's all you can be bothered doing. I hate doing the shower too, but once it's clean, it's much easier to keep it clean on a regular basis. And, it doesn't take too long either.

We use shower power (it doesn't smell too bad), and the kitchen non-scrath scourer. Every few weeks I guess...

Good luck.