Communication is key. Collect customer information, e.g. SMS or email, and reach out to customers frequently to keep them informed about your brand, new products, events, and more.
Allow customers to decide which channel they'd like you to reach out via and consider using marketing automation software to do so accordingly. Make sure the software lets you switch between email and SMS at scale based on the specific customer segment you're communicating with.
Create value across your communication strategy. Give customers something in return for signing up to your newsletter, for instance, by implementing a loyalty program.