A popular formula that some of the top marketers use is sending 3 relationship building e-mails to every 1 sales e-mail. This is especially important in the beginning stages, and it will set the tone for later emails.
What you might want to do is have the first e-mail be entirely focused on offering quality information and building a relationship, the second e-mail making a very soft pitch, the third e-mail selling a little harder, and finally the fourth e-mail blatantly promoting a product.
If it makes you feel more comfortable having a system to work with, then this is a sequence that you may wish to follow.
Another system that may work for you and your subscribers: 4 ABBS – 4 emails that are “Always Be Building AND Selling”.
Put another way, ALL of your emails should be relationship-building, offering quality content that is actionable and achievable. Inside each of those emails is a “pitch” of varying degrees. Most of the time, those “pitches” are just links in your email signature or your PS. One out of three is a blatant “Get this because it will help you and here's why” pitch.