Once you are on the journal homepage, look for a link to "E-Alert Sign Up" or "Issue Alert." Some publishers will ask you to create a username and password, while other publishers simply ask you to provide an email address.
HighWire Press Alerts
Use the My HighWire Press feature to create eTOC Alerts and CITETRACK Alerts for journals published by HighWire Press.
Science Direct Alerts
Science Direct offers a variety of alerts for journals published by Elsevier. After creating a profile, you can create Search Alerts and Volume/Issue Alerts.
The SpringerLink site offers Table Of Contents Alerts for Springer journals.