The Numbers With the Subscription Form

After writing those numbers down I inserted a simple email subscription below every single post. I also included a small message explaining to the visitor that they could subscribe for free, and that we would keep sending related tips to their email inbox. You can see the form live below this post as well. Here is how the numbers behaved on the following weeks.
DailyBits.com

• Monday of July 28 = 114 email subscribers
• Monday of August 11 = 119 email subscribers (+5)
• Monday of August 18 = 122 email subscribers (+3)
• Monday of August 25 = 125 email subscribers (+3)

The previous average gain was 1,3, and after I inserted the form it jumped to 3,6.
DailyBlogTips.com

• Monday of July 28 = 979 email subscribers
• Monday of August 11 = 1025 email subscribers (+46)
• Monday of August 18 = 1050 email subscribers (+25)
• Monday of August 25 = 1084 email subscribers (+34)

Without the form this blog was getting, on average, 15,6 new email subscribers every week. After I inserted the form the number more than doubled to 35.
DailyWritingTips.com

• Monday of July 28 = 5153 email subscribers
• Monday of August 11 = 5245 email subscribers (+92)
• Monday of August 18 = 5310 email subscribers (+65)
• Monday of August 25 = 5373 email subscribers (+83)

On the third case I noticed a big jump as well. The previous average was 55,6, and after the insertion of the subscription form it jumped to 80.
Conclusion:
I think the results are pretty clear. By simply adding an email subscription form below each of your posts you could reap many more email subscribers. Consider Daily Blog Tips for example. Without the form, in 6 months, I would have 374 new email subscribers. Using the form, however, this number would jump to 840 subscribers.
Obviously the effectiveness of this technique will depend on a couple of factors. Tech oriented websites might see smaller results, while blogs with a more varied audience will see better results because those visitors are more likely to subscribe via email (as opposed to RSS feeds).
The styling the overall layout might have an impact also. If the bottom of your articles is cluttered with all kinds of badges, links, social bookmarking icons and the like, the email form might get lost among those and not be effective. If you have a clear design, however, it will be easier to make visitors notice.
Regardless of those variations, I think it is definitely worth a try.