I think your biggest indicator is who your fellow passengers are. I get upgraded a lot more on FRA-HEL or MUC-HEL than other routes from germany. I have started to think that since most of my finnish fellow coworkers are using SAS Eurobonus cards then that means there are fewer SENs onboard and my SEN card does the trick.
Same probably goes for long distance routes. There are probably plenty of SENs going on holiday to Bangkok but not that many to Caracas.
From looking at the expert flyer seat map I can even see that sometimes i'm upgraded even though there are one or two empty seats still in Y.
But I have seen a couple of times recently people refuse op ups since they are traveling with non upgraded partner. In that case someone else (often the standby pax put in the upgraded persons seat) that gets the op up.
So based on this logic you should always board last. Hope that someone else is in your seat and you'll get their op up !
One guy even ignored the new boarding pass for C and sat in his original seat next to his wife in Y. The woman who boarded later and found him in her seat got an op up to C in the plane.