Pregnancy brings about the craziest and most erotic sex filled dreams! (although pregnancy isn't a good reason just for wanting more sex dreams :lol:
I think it goes along the lines of the urge to go to the bathroom tips, as it's putting extra pressure upon your genitals. Although I know alot of pregnant women just can't even stomach the thought of sex, that it seems to kill their sex drive for some reason. Not me though. :D
But I think the best way would be to think about it as emotionally involved as you can possibly get just before falling asleep. If it's something that's constantly on your mind, it's more than likely to show up in some form or another in your dreams.
Just be warned. Dreams are unpredictable and you might find yourself with a washing machine instead of a person!