Well first I would change the location of the Slim. Maybe it's in a location such as the floor or near a fan which will attract a lot of dust. Try wetting a cloth and gently scrub against the 360. Cleaning the Slim is like cleaning any glossy material There should be websites that provide more information (google your question, lol) Anyway while your at it. with the 250 GB of HDD you have, install your games to your HDD. faster load times, doesn't scratch disk, plays a scratched disk, quiets system.
There is no way to clean the Xbox 360 slim 250GB that does not get it scratched. I have had original Xbox, and two of the old Xbox 360's, no problem, but the new slim 250 GB is bad for scratching. I have used feather duster, and soft cloth, but nothing helps. I wish I had gotten the 4GB with the matte finish, but that's hide sight. The new slim is great and I traded a PS3 slim to gamestop for the $200.00 credit and then paid the difference. My only recommendation is for people to buy the 360 slim 4GB with matte finish and then get the hard drive later (if scratches matter to you). They would not matter to me except they I have the Xbox on my desk and can see it all the time. If I had it in an entertainment center and could not see it, it would not matter so much. About all that can be done is to dust it and not worry about scratches and wait until somebody comes out with perhaps a skin to cover the top. The huge fan on top (if you have it flat) is a dust magnet, but will hopefully keep it from the problems of the older Xbox 360's. I have had no problems with the 360 slim, and it is the best gaming console there is, the only problem is the scratching of the surface, which is only a problem to neat freaks like me..
Here's what I do. When my systems cooled down and when I'm not playing, I cover them with a pillow case. Make sure you cover it with something that's lint free and such. That really helps to keep the dust away and makes cleaning much easier.