First, use your web host's built-in analytics tools to determine what the most frequently accessed files are and what the largest downloaded files are. Webalizer is typically available.
Make sure to enable gzip compression in your htaccess file.
AddOutputFilterByType DEFLATE application/atom+xml \\
But, the bottom line is that since your site offers downloads, those files will likely be the largest part of your bandwidth usage.
If you offer your downloads as image files, try offering them as zipped files instead--that might save on some bandwidth.