localhost and relative paths - apache

Hi,
I have Apache running and i'm tring to get my website running on my localhost before I push it to my live server but i'm unable to see some of the javascript and css files with relative url paths.

for example:
if i go to my index page
http://localhost/~username/mywebsite/index.php
when i try and use the relative path for the css file: /includes/css/main.css

it thinks my css file is at this path
http://localhost/includes/css/main.css
but it is actually supposed to be at this path
http://localhost/~username/mywebsite/includes/css/main.css

does anyone know what settings i need to change in order for the relative paths to work on my localhost.

thanks