If you want to make sure your team reads all of the announcements, you need to set the example and read them first. Remember this saying: "If it is important to you, it will be important to them."
When the announcements are made, you should put a link to the announcement on your stream posts, in your team mail, on E-Cards, and your leadership page. If your team sees that you are excited about the announcement, they will understand how important it is to read it.