If used correctly, a popup is always the best way to achieve great results (be it in sales or subscription rate). It is also the best tool to grab your readers attention.
For a wordpress site, there are several plugins available. I choose the wp-super-popup plugin. Install the plugin from here: http://wordpress.org/extend/plugins/wp-super-popup/
When the plugin is installed, you will see a “super popup” menu in your admin panel, click on it and there you will be able to modify some settings.
To make it more user friendly, I decided to explain every setting and what there are used for in those printscreens: