I was actually looking for a way to do this yesterday.
If you're doing this with something like WordPress, you'd want to use their documentation and not necessarily CSS. They have tags you can add separators to.
In CSS, you can do this with probably some padding:
PHP:
li + li:before {
content: " | ";
}
And it won't put the "|" at the beginning. This doesn't work in IE6 though (if that is an issue for you).