mod_rewrite:

# Redirect to example.com -> www.example.com
RewriteEngine on
RewriteCond %{HTTP_HOST} ^example.com$
RewriteRule ^(.*)$ http://www.example.com$1 [R=301,L]

# Redirect www.example.com -> example.com
RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.example.com$
RewriteRule ^(.*)$ http://example.com$1 [R=301,L]