Post order modifications

I am looking for some pointers to ways I can modify the order of posts. What I am trying to achieve is an integration of news items and announcements:

• Announcements are short items taking place at some point in the future, tagged by date (posting date) and Category.
• News items are regular posts, ordered by date.

What I am trying to do is have the next forthcoming (i.e. in the future) announcement show at the top of the page, followed by the most recent news item, followed by either the remaining announcements or a mixture of these and the remaining posts. If anyone has done something similar to this, or can point me in the right direction, I would really appreciate it.