I have a number of EC seat reservations cards in my bag and pop one of those onto the seat beside me. Most peeps looking for a seat won't (until the train is rammed) examine every reservation.
Also (leaving KGX) there's always 10-15 'last minuters' who walk through FC and then sit down in the 1st seat they see in standard. Best to avoid G/F and sit in D, which they rarely reach.
I find the 'quiet' coach B is often the least full, maybe peeps think you can't use iPods etc. and avoid. You can, with 'consideration'.
Sit in the aisle seat, leaving window seat free until the train is out the station. That obliges peeps to stop and ask you to move and encourages them to find a seat they can just drop into. If they do make you get up, make a point of pushing the armrest down between you.
Wear a comedic tartan tam-o-shanter and heap up cans of McEwans Export on the table (obviously this only works on services in England)