Chances of being downgraded are low, however the following things help
1- have a seat selected
2- check in as soon as possible
3- Select your meals via Q-eat ( if on your route) 7 days before travel
4- If you have status that will help ( ie a silver would be kicked off before a Gold if all other things are the same like fare type)
Things they look for when downgrading a passenger - It is a combination of one or some of the below
1- have they checked in ?
2- are they on a connecting flight inbound ? ( often flights can be delayed resulting in miss connect )
3- TCP - They do not like to change travelling parties
4- fare type - a full J fare will have preference over a discount or reward fare
5- Airline status - the higher you are the less chance of getting downgraded
6- booking date