When adding/editing a historic post – e.g. write up of a competition from a few weeks ago – use the Publish option on the right hand side to set the date of the post.
Click the Immediately link to show a calendar popup and select the date for the post – e.g. the date of the competition.
Doing this will show the post on the news page in the correct date order with respect to other posts. However, this may mean that people miss the new “historic” posts. In this case, and especially if several historic posts are added at once, create a new “new posts summary” type post to link to the historic posts.