Is there any way to prevent the grills from getting dusty? I generally put my iPhone in my pockets with no keys. Scratches are not a problem because I don't put anything in the same pocket as my iPhone but the speaker and mic grills on my 3GS are extremely dirty. Even the little nook where you slide the mute switch has dirt in the crevice. Does using a case minimize this or is dust collecting in those spots inevitable? I thought about using a can of compressed air but that only blows the dust into the phone.