I work with multiple files a lot, so I'm always navigating between split screens and across buffers.

  " Split windows/multiple files
  " use <Ctrl>+s to split the current window
  nmap <C-S> <C-W>s
  " use <Ctrl>+j/<Ctrl>+k to move up/down through split windows
  nmap <C-J> <C-W>j
  nmap <C-K> <C-W>k
  " use <Ctrl>+-/<Ctrl>+= to maximise/equalise the size of split windows
  nmap <C--> <C-W>_
  nmap <C-=> <C-W>=
  " use <Ctrl>+h/<Ctrl>+l to move back/forth through files:
  nmap <C-L> :next<CR>
  nmap <C-H> :prev<CR>

Note these use the same 'hjkl' navigation keys.