I’ve always hardcoded the domain name in my htaccess’es, requiring me to make changes each time I deploy a new website.

The solution:

Behold, an alternate, generic method of redirecting non-www to www and www to non-www, requiring no changes between deployments!

Non-www to www

RewriteCond %{HTTP_HOST} !^www\.
RewriteRule (.*) http://www.%{HTTP_HOST}/$1 [R=301,L]

www to non-www

RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^/(.*)$ http://%1/$1 [R=301,L]

Bonus tip: Remove trailing slash from address line

RewriteRule ^(.+)/$ http://%{HTTP_HOST}/$1 [R=301,L]