It can be difficult for people to find a way how to follow your blog and losing people interested in reading your blog is not something you can afford. Although you can customize a follow button in the side bar of the blog, it can still go unseen. A pop-up should be up on your blog after the viewer has spent around 5 seconds. I use MailChimp to organize my pop-up. Keep the pop-up minimalistic, not loaded with information or pushing in about the blog. Make the viewer comfortable knowing that he or she has a choice of either accepting or rejecting the subscription. Having a choice works in a positive way.