Backs are incredibly complex structures with a vast amount of things that can go wrong, that multiplied by our own physiology probably means that no two 'bad backs' have ever been the same. Hence millions of different remedies, all of which will all work for someone somewhere - finding what helps you is just trial and error. what cures one will make someone else worse.
Very gentle 'beginners' pilates is what helped me the most. And I find cycling to be excellent too. As for mattresses, I like mine hard. But there are plenty who find cycling to be a bad thing and plenty others who swear by a soft mattress.
Probably keeping yourself lean and fit are very important, but even then i'm sure that won't be ideal for all.