Bear in mind the airline needs a reason to upgrade you - they are not going to do it just because you ask unless they have a reason of their own for doing so.
In no particular order:
1. Join the airline's frequent flyer club and build up a good customer record. (keeping you happy for continued repeat custom)
2. Be travelling on a full fare economy ticket rather than a cheapie. If they need someone to upgrade it's more likely to be you.
3. travel on your own and be reasonably smartly dressed (ie you will 'fit in' with other clientele)
4. Be polite. If you are an awkward character they won't want you upsetting their higher paying passengers.
5. Travel at a time when few businessmen are travelling but lots of leisure passengers. If they are overbooked in economy they will fill up empty seats elsewhere
6. Check in late but not too late. (see 5)
7 (sure fire this one - 95% or more of the time the others are, let's face it, just not going to work) just book and pay for an upgraded ticket in the first place
