Get URL Rewrite

Install URL Rewrite from http://www.iis.net/downloads/microsoft/url-rewrite or the Web Platform Installer.

Configure it

Merge this into your existing web.config, and all traffic will be redirected to HTTPS.

<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="Redirect to HTTPS" stopProcessing="true">
                    <match url="(.*)" />
                    <conditions>
                        <add input="{HTTPS}" pattern="OFF" />
                    </conditions>
                    <action type="Redirect" url="https://{HTTP_HOST}/{R:1}" />
                </rule>
            </rules>
        </rewrite>
    </system.webServer>
</configuration>