Be clear in your email what they can get out of subscribing to your SMS list. There should be a clear difference between having only an email subscription (definitely a valid option!) or changing into an SMS subscriber (or maybe even both!).

Whatever your offer is, make sure you clearly state what's in it for them.

You should let them know how often you plan to send messages, and what kind of messages they can expect to get. Don't tell them you'll get them exclusive deals, but they later find out it's open to everyone. Or that you'll send them messages twice a month but they end up getting a message every week.

SMS is by nature more intimate than email marketing. Don't betray their trust in you by spamming them and ignoring expectations.

Don't make things harder for them to sign up. Give them different ways to sign up -- keyword shortcodes work, but make an online method available to them as well. After all, 46% of emails are opened on mobile phones, so they're likely to be able to quickly sign up with a form rather than typing out numbers on a text message.

Not everyone will want to move into an SMS subscription list, and that's okay. What's important is that you are able to provide a better experience to those that do, without harming the experience of your other customers.

If you have a clear benefit to "upgrade" their subscription, set expectations, and make signing up as easy as possible, you'll get subscribers who are excited to be on your SMS list. Build on their experiences and move forward.