Website: ProjectManagementHacks.com. Currently using the SumoMe plugin for sign-ups which is working moderately well and has the virtue of being free. I'm willing to look at paid plugins (prefer one time fee to monthly recurring) if there is good data & case studies showing the value
I personally use Subscriber's Magnet by MaxBlogPress. This is a one-time fee plugin, but it allows multiple points of engagement (e.g. top of page, bottom of page, sidebar, popin, comment box, etc...) to increase overall optin rates for your site.
My site saw a 32% increase in subscribers after deploying this plugin.
I would also consider a design update as well with "Calls-to-action" that are clear and easy to spot. I love and recommend optin monster (https://optinmonster.com) mainly for A/B testing, multiple types of optin variations from lightboxes, bars at the top/bottom, after post, slide in and my personal favorite is their exit-intent. Exit intent allows you to show a lightbox as the user is leaving to push something one last time.
You can try something like Hellobar or ManyContacts
Here are some thoughts on a foot in the door technique I wrote on my blog
On WPMayor.com we use Optin Monster (http://wpmayor.com/link/optinmonster). Formerly a plugin but now a SAAS, it can function on any website, not necessarily a WordPress one.
It's got tons of features including split testing and ready-made design templates. You'll see the best conversion rates when you offer a compelling proposition to site visitors. A PDF guide tends to work well.
To take it to the next level you can try content upgrades.
Syed Balkhi (CEO of Optin Monster) offers a great explanation on how this works:
"With Optin Monster you can also offer different Content upgrades are lead magnets (or opt-in bribes) that are created specifically for a particular blog post or page.
It replaces your generic eBook or “get free updates” offer with something much more targeted that strongly appeals to the person on that page."