1) Movable "walls". Do you have a dresser? A headboard? In a single room apartment, they can serve as a room divider. Even your dining table can separate the kitchen from the bedroom - perhaps with a bench seat on one side instead of chairs. If you have a budget, use folding screens. Second-hand windows, doors and shutters can make a great screen depending upon your style. For the do-it-yourself expert, you can build a divider from shelves, blocking off some of the openings for more privacy. If a wall is more your style, build one that can be removed, inserting tempered glass or plexiglass to keep light moving through. To separate my kitchenette from the rest of the space I had my dad build a tall cabinet then added a top shelf and a table at 28" that attached to the wall. It kept the space open, added great storage and I eliminated needing a full table and chair set. While they are expensive, room dividers that revolve storage and the TV from one side to the other, are a great solution.
2) Let the light in. Privacy is key in a big city, but small spaces NEED light. A transparent or open-weave room divider can keep your bedroom or guest sleeping area private from the public space. Leaving windows un-dressed (no drapes or hard treatments) using window film is another option and easy to install. For simplicity use frosted effect films, or for drama use patterns and textures. Literally add light to cabinets, shelves, moldings and surrounding art and large pieces of furniture: LED strips and ropes provide light without high energy use!
3) Uniformity is best. To make small spaces seem larger, unifying the color scheme is best. Monochromatic color schemes in neutral colors will help make walls and floors seem larger. Accent the space with colorful artwork and large scale accessories. If you have many collectibles, group them in one area to keep the space looking open.
4) Store it. Having a clutter-free space is attractive as well as provides peace of mind. For that dual-duty kitchen table/desk, find some great wicker or paper boxes with lids and
5) Open it up. In apartments with many small rooms or long hallways with rooms off to the sides, removing doors (store under beds if you can) is a solution to keep spaces flowing together. Are you an owner? Try opening a wall and creating a pass-through or even add glass for a window effect. Here, the wall was partially removed and stained glass insets added for privacy and a fabulous artistic element.