When you create or edit a blog post, there's a panel on top of your tool bar where you can enter additional information for your post. Click to open it.
In order to keep a news item on top of all the others in the Corporate or Personal News Feed macro, check the "Sticky news" checkbox. It will automatically open a date picker field this field is mandatory, you cannot save your post without one. This date determines, when the post you are editing will lose its "sticky" status automatically. By default the date is set to two weeks in the future, but you can change it to any future date you like. If you want to make your post sticky forever, simply set a date in the next century
You can edit your post and uncheck the checkbox any time. The news will then lose its sticky status immediately upon saving.
The mandatory date makes sure, that you will not have dozens of sticky posts to clean up manually when they clutter up your feeds too much.