so far i just use an external HD, but i'm seriously considering looking into cloud solutions or similar, if only for my work archives. i might slip my ebooks in there as well.
as for specifically my ebooks, from time to time i just copy the entire calibre folder over to the other HD.
pros of this method : easy, costs only the price of a hard drive (not much).
cons : depends on you remembering to do it, so is unreliable, and not the ideal solution for maximum security since it's right next to my computer, so in case of theft or flood or whathaveyou i could easily lose the main drives *and* the backup in one go.