Tubes are so weird - it seems like you are either in a carriage of lovely people and get offered 3 seats at once, or else in a carriage of total bastards who sit there, noses in papers, steadfastly refusing to look at the huge bump towering over them.
I was in a carriage of bastards this morning and went to work grumbling about what arseholes Londoners are, and then had a carriage of lovely people on the way home, and felt all warm about the world!
Strange to say, 9 times out of 10 it's a woman who gives up her seat - I never thought men were so selfish until I got pg - I think some of them positively resent you having a "right" to a seat.
I agree about strategic rubbing of the bump - also a good deep sigh helps, plus fannign yourself as if you are feeling incredibly hot and faint (well, I say "as if" but let's face it, that's most tube journeys, isn't it?). But I've never had the nerve to ask someone to get up for me - although other passengers have done it for me several times (asked other people to move, I mean).