What is the best way of getting the upgrade from Economy to Business?

I have a holiday booked for 4 people, heathrow to Dubai, travelling at 9am flight on 30 October to 3.15pm flight on 5 November 2013. We all have the required miles for 1 leg on business.

My parents were on the 8.40am flight this morning heathrow to Dubai, but no upgrade flights came up the night before and a week before, checked everyday, even though there were plenty of business class seats available.

I suppose it is rare to get an upgrade?