There are three main types of alerts you can set up:
Table of Contents Alerts.
Have the most recent Table of Contents (TOC) of a journal sent to you via email or RSS feed. TOC alerts are often available through the journal publisher's website. Also, some databases that index the journal offer this service. And there are free aggregator services, such as Journal TOCs that allow you to sign up for multiple alerts via one account. (If you are not sure of top journals in your field and your research is in the areas of science, technology, and social sciences, consider using Journal Citation Reports to evaluate and compare journals using citation data and article counts drawn from over 21,000 scholarly and technical journals. You can also consult Ulrich's Periodicals Directory to identify journals through subject and keyword searching.)
Saved Search Alerts.
Have a search consistently or periodically run in a specific database. You will then be notified when new articles are added to the database that match your search parameters.
Citation Alerts.
Be notified when a key citation in your research has been cited in another document. This can be set up through ISI Web of Knowledge and other databases.
Most alerting systems allow you to choose how often you want to be "alerted": on a daily, weekly, or monthly basis.