If you have Vim >= v7.3, you can simply add this to your .vimrc to highlight 81 and onward (so 80 is your last valid column):
let &colorcolumn=join(range(81,999),",")
If you don’t see a highlight, you may not have a ColorColumn highlight color set. Add this (adjust to suit your preferences):
highlight ColorColumn ctermbg=235 guibg=#2c2d27
Now I like to highlight column 80 as well as 120 and onward, so I have separate “warning” and “danger” markers. You can do that thusly:
let &colorcolumn="80,".join(range(120,999),",")
Example
Here’s a screenshot of GVim editing my .vimrc.

I use Ubuntu Mono 11 font and the molokai color scheme. You can find my vim config and all of my dotfiles on GitHub.