Today's "theme" is wall decor.. Often, in apartments, you're limited to the wall color that the building decides... Not to fret, there's TONS of things that you can do with your white or beige walls to make your place a little more like yours rather than everyones.
Bombay Outlet online has some great deals on wall decor. You'll find things in this online shop that may be discontinued in the stores, overstocks or other fab finds. The grill on the right is about half the price you'd pay in "fancier" department stores. The tapestry on the right is only $16! With large focal pieces like this, beware of overkill. Use smaller sconces to surround the piece or hang two similar tapestries next to each other. But don't try to fill your entire wall with pieces like this. Too much of a good thing, just looks cluttered.
Try arranging a group of smaller framed photographs or prints on facing walls like the example on the left. The art is available through Homevisions.com along with several other examples of this type of arrangement. Arranging art in this manner will make the room look larger. Try this instead of slamming your desk or bookshelf in the corner of the room.

Novica.com is a diverse and intriguing site that offers incredible home decor, jewelry, musical instruments and much more (they even have bean bag chairs!). Novica offers art work from over 2000 extra-ordinary master artists from around the world. You can shop by region (West Indies, India, Mexico, West Africa, Thailand, Brazil, Bali, Java and the Andes) or by category. If you have a favorite animal and want to theme your room, you can even see all the items available that are related to, say, elephants (they have over 430 elephant items!). They also have a painting selector tool that will help you locate art work by a style, region, subject, price range, dominant color or size. Its a great way to shop, plus you get to read about the artists and the areas of the world where they create these gorgeous works.
There are amazing things that you can do with lighting in an apartment... Create the illusion of a larger room, set a mood, compliment a theme. From candles to christmas lights, here are a couple things to think about for your walls.
From Illuminations.com the pic on the left shows what can be done
outdoors with their candle chains. But don't let the lighting stop there, sure this would look great on a porch or balcony, but if you don't have one, you can also use this item around the perimeter of your kitchen door or front door. Save a little electricity (and some cash) by using these in dark hallways or in the bedroom. At right, also from Illuminations.com, a focal sconce with candles. This would be a great piece for a bathroom, yes, a bathroom... Keep other wall hangings to a minimum with this piece as the main focus in a room. And of course, never leave a lit candle unattended.
Mirror, Mirror, on the wall.... It truly is amazing what a little light and a mirror will do for a room. LightingDirect.com offers a huge selection of light fixtures, indoor and outdoor and has a pretty good supply of hard to find lighted mirrors. Hang one of these at the end of a hallway, it gives an impression that the hall is longer than it really is. 
Spiegel online also has a great selection of wall decor at reasonable prices. Items at Spiegel are constantly changing and going on sale so you'll need to check back often. Right now, they have some great examples online of grouping mirrors as well as using mirrors to make a space seem larger than it actually is.
Possibly one of the sweetest finds I have ever found.... You can get these at a handful of places but Linens 'N Things has them online and at one of the lowest prices. For $9.99 each these record
album frames are not only great wall decor but great conversation pieces. I'm sure you're dad won't part with that first edition Bob Dylan, but sadly, there are a lot of record shops going out of business lately. (Just last weekend, I ran into a shop that had vinyl for sale at 4 for $1! Luckily, I've still got my "vintage" Duran Duran, Echo & the Bunnymen, Psychedelic Furs and Pretty in Pink vinyl in original, perfect condition! Of course, that didn't stop me from snagging some quality Oingo Boingo and Peter Gabriel...) Hang these frames in groups. In a straight line above your sofa. Arranged as a frame around another music themed poster. The beauty of this frame is that you can totally make it your own and show off your great taste in tunes!