I have an array of post ids which I'm passing to query_posts using 'post__in'.
My question is how can I use the order from my array rather than ordering by 'date' etc?
You would need to build your own query and pass it to query_posts i.e.
if your array of post ids is:
then you can query this way:
This will give you a result set in the order of your post ids array.