When you're killing (C-u, C-k, C-w, etc) + yanking (C-y), you can also use yank-pop (bound to M-y in bash and zsh by default) to replace the thing you just yanked with the thing you had killed before it.

  $ asdf<C-w>
  $                  # now kill ring is ["asdf"]
  $ qwerty<C-a><C-k>
  $                  # now kill ring is ["qwerty", "asdf"]
  $ <C-y>            # "yank", pastes the thing at the top of the kill ring
  $ qwerty<M-y>      # "yank-pop", replaces the thing just yanked with the next
                     # thing on the ring, and rotates the ring until the next yank
  $ asdf