A friend of ours just moved into a pretty small studio and wants to have her bed, a sitting area, a place to play music and then room for books and media without it looking like a closet crammed to the gills with stuff. Also there's really not space to use a bookshelf as a divider. It's a tall order but we think (thanks to this example) that with a curtain divider sectioning off a 'bedroom' she'll have the feeling of more space:
We love this idea of hanging one drape along a side of the bed. It doesn't have to be a giant floor to ceiling curtain to give the illusion of a separate sleeping nook. Found this image from dnaisjustcmyk.
For a seamless and less cluttered look in a small space, consider using hospital track to hang your divider. It will ensure that you can sweep the curtains open easily when you want to, and they'll hang evenly all the way from the ceiling. Image is from commendaces. For more info on hospital track systems, check out this post.
You can also go the route of just giving the illusion of a doorway by adding curtains to either side and leaving an opening. Also love the natural linen look of these dividers. Found the image on apartmentsasha. And if you have a minimal space and want to try something a little bit more design-y, take a look at these cool magnetic curtains that could be used as dividers.