Preference usually goes to top tier frequent flyer account holders. After that it is a little bit of pot luck really. Tips I would give are look presentable, travelling alone helps and sign up to their frequent flyer accounts.

The other tip is ask and ask politely.

Even if a complimentary upgrade is not available I have purchased upgrades for next to nothing. I flew back from Jamaica and upgraded from economy to first for $250. I still had to eat my economy meal but a bargain in my opinion!

Hope that helps.