Redirect non-www to www in .htaccess

RewriteEngine On

RewriteCond %{HTTP_HOST} !^www\.

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

For Https

RewriteCond %{HTTPS}s ^on(s)|

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

Here's the correct solution which supports https and http:

# Redirect to www
RewriteCond %{HTTP_HOST} ^[^.]+\.[^.]+$
RewriteCond %{HTTPS}s ^on(s)|
RewriteRule ^ http%1://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

UPD.: for domains like .co.uk, replace

RewriteCond %{HTTP_HOST} ^[^.]+\.[^.]+$

with

RewriteCond %{HTTP_HOST} ^[^.]+\.[^.]+\.[^.]+$

Change your configuration to this (add a slash):

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

Or the solution outlined below (proposed by @absiddiqueLive) will work for any domain:

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

If you need to support http and https and preserve the protocol choice try the following:

RewriteRule ^login\$ https://www.%{HTTP_HOST}/login [R=301,L]

Where you replace login with checkout.php or whatever URL you need to support HTTPS on.

I'd argue this is a bad idea though. For the reasoning please read this answer.