I get a really packed train home from work and there are usually quite a few people standing.
Despite this, some people who have a seat obviously think they've paid for a seat for their bag too. They don't even lift it when other people clearly have no seat.
Whereas other 'standers' just leave them to it, I just go up to them and politely say, "I would like to sit down; can you please move your bag."
They usually look really pissed off. It's worth it for the look alone.