I too wanted to "de-college" my apartment when I graduated. One of the things I did that helped a lot was buying original artwork. I would go down to Portland's saturday market on a regular basis and try to find stuff that was unique and cheap. I picked up lots of pieces between 10 and 50 bucks (I had to get a lot of them framed though). It doesn't need to be expensive. Having unique stuff that most people haven't seen before goes a long way towards giving your place an "adult" and original style.
Another good inexpensive way to really set your aparment apart is to buy a few antiques. For example I bought an antique wardrobe. It's made from hardwood, richly stained and looks really good. I bought it from a place that imports and restores Antiques from England. It cost me only 200 dollars. Compare that to the latest Ikea catalog where similar sized wardrobes start at 250 dollars and don't look nearly as good and don't have nearly as much character.
One last thing, have something other than beer and college-style liquor around (you know, Malibu Rum, Jack Daniels, cheap taquilla, stuff like that). You can quickly accumulate an asortment of interesting wines at a place like Trader Joes that aren't very expensive. Have some good gin, good taquilla, and a decent bottle of bourbon or scotch. on hand as well.
On a related note, for what it's worth, I found that at that age having a "de-colleged" apartment
really helps with dating. It sets you apart so much from other guys in their early 20's. With my old apartment I quickly noticed that the girls I had the most luck with were ones who had seen my place. So much so that, eventually my main "first date" strategy evolved to take girls out somewhere in my neighborhood (downtown Portland is nice in that there are lots of cool places to go.) so that I could think of some excuse to take them by to see my apartment. "Oh crap its raining, don't worry my place is on the way to the next bar. Let's grab an umbrella really quick"
