The problem with upgrading at check-in is that you may not get the seat you want and you miss the chance to order your meal online (as I'm sure you know). The vouchers only last a year and with my having a number of airmiles expiring earlier because of the shortened expiry, I'm using them asap.

I've not had a single OP-UP since being Diamond - I've flown in Economy and Elite as well as PL.

I did have a number of OP-UPs as Gold from Elite to PL. All I can suggest is you look for flights where there is no Elite availability and use the 72-hour guaranteed availability to get one.