There is an op-up experiences thread which will give you some idea of other's experiences. As a Skywards Blue Tier member I would not get your hopes up too high - in general members on higher tiers will receive an op-up first (although there is a debate about whether being Platinum helps or not). There has been discussion about preference being given to those with a surname beginning with a letter at the start of the alphabet......

In general the advice given in the past about dressing up, smiling at the check-in staff etc probably doesn't help nowadays when computer systems hold loads of info about passengers and their worth to the airline.

Having said all that - occasional surprises happen so good luck.