In this article Jordan Ayan provides these 7 tips for your welcome messages:

1. To start, html is a standard. Your welcome email should be a
   well-designed html message sent out in multi-part mime so that it will
   display properly in text or html. It should also be optimized so that
   if a recipient’s images are “off,” it will be legible, understandable
   and eye-appealing.

2. It should thank the recipient for providing the required
   information. It should also reinforce the value of your emails,
   reminding them what to expect in terms of frequency and any other
   information that reinforces relevancy to the recipient.

3. It should include the information recipients need to add
   you to their white or “favorites” list, including the “from” address
   for your message.

4. It should reinforce the value you place on the
   relationship and reinforce your privacy commitment, including a link to
   your privacy statement. If you don’t plan to share their email address
   – restate that fact clearly.

5. A little something special. The subscriber has expressed a
   high-level of interest, so this is the time to offer something extra as
   a thank-you: a coupon, a percentage off, free shipping, a bonus
   download of a white paper or a free webinar. Just let subscribers know
   that you appreciate the fact that they supplied their email address,
   and the trust they are placing in you by signing up for your message.
   You can test offers to see which ones are most effective.

6. If you use a double opt-in process, you have an
   opportunity to double-dip. The initial confirmation message can include
   a promotional message. This should be muted from whatever you would
   send in your welcome message once they confirm.

7. A welcome message should
   appear in the subscriber’s inbox within minutes of when they sign up.
   Wait too long, and recipients may forget who you are or why they
   subscribed.

Source: Email Insider