The first is from DHS staff Darrell Cason. He emails alert notices as they become available; get on the list by emailing Cason. And a shout-out to Cason for including the Shelter Hotline number (1-800-535-7252) in all alert notices.
The third way is to check the Department of Human Services website, dhs.dc.gov. The website is updated automatically but you have to remember to check it.