
Have you ever relented and gone into her at night when she has been barking and crying? If the answer is yes, then she *knows* that if she keeps it up, you *will* come ;) You have to be very firm and decide that, if you want her to sleep alone, you will NOT go into her at all.
Keep her awake more during the day, play with her, take her for a few short walks, do some basic obedience. If you fill her brain with the stuff during the day, she is more likely to sleep at night.
Make sure you leave her on her own for just a few minutes at a time during the day, don't make a fuss of her when you go back in to her, just carry on as usual . Don't make a fuss when you leave her either, just go out of the room for a couple of minutes. She will, hopefully, soon learn that nothing bad happens to her whilst you are away
Good luck :)