As of vim 7.3, you can use set colorcolumn=80
(set cc=80
for short).
Since earlier versions do not support this, my .vimrc
uses instead:
if exists('+colorcolumn')
set colorcolumn=80
else
au BufWinEnter * let w:m2=matchadd('ErrorMsg', '\%>80v.\+', -1)
endif
See also the online documentation on the colorcolumn
option.