If you have ever got an upgrade on a flight.....

[bramblina]

.....did you already have seats pre-booked?
I've just read that a way to get an upgrade is by NOT pre-booking your seats. If your seats ARE pre-booked, they cannot be changed.
DH and I are going to NYC for 6 days sans children in December and as this is likely to be a once in a life time (and I'm only talking about the child free trip here....) I would love to try for a cheeky upgrade.

Any expert flyer tips.......?

OP how about you just buy the business or first class ticket and know from now that you'll get to sit in the nice seats. It's really not rocket science.

And have you any idea at all that right now there are hundreds of thousands of people up in the air going on a special trip? What makes you different?

We get upgraded relatively often on short haul. My husband is a gold card holder with BA & I'm silver.

On long haul with a legacy carrier (BA, AA etc) it's very unlikely. We only ever find out at the gate because the flight is overbooked.

BA - LHR often offers sensible priced upgrades (esp economy - prem economy) when you check in.