For us it ends up being roughly where we want to go and roughly when we want. We like open jaw, going out to one place and returning from another, and for a road trip can be fairly flexible on return airports but usually book well in advance due to availability not lasting long on popular destinations. If you are really flexible, seats do come up at short notice and you can get alerts from various sites about seat availability.
Depending on flight timings (day/night etc) we like first class outbound, to take advantage of all the LHR comforts and wine and dine on the aircraft during a day flight, and then collapse into a hotel at the other end. Not so bothered about first for the return, specially where there is less to be gained at the airport, probably want to sleep if it is a night flight and probably no drinking to be done because we have to drive home from LHR.
In terms of 'efficient use of avios', people will work out pence-per-avios, but if it is somewhere you want to go, and/or an experience you want to have, it will be great value anyway - even more so if you wouldn't have paid for it otherwise.
Edit: Good points Bertie - amazing how many other posts arrived during my snail-pace typing!
Last edited by vintagepilot; May 17, 2018 at 2:03 am