Keep your contacts updated. Ensure that any change to your contacts in Xero are reflected in your mailing lists within Mailchimp automatically. A set of default mappings ensure that the information contained within your 'Contact > Contact Persons' is set to the correct merge fields in Mailchimp. All you need to do is create the Mailing List within Mailchimp and set it correctly in the installation steps, this integration will handle the rest.
Additional Information & Help
Xero + Mailchimp Integration
Zapier: Mailchimp + Xero Integrations