I have always had a wall unit type entertainment center and placed the center speaker above the TV but now I have a simple flat TV/AV stand. My solution is to mount a shelf on the wall above the TV. You could build a shelf, use a TV wall mount, or on-wall speaker mounting bracket. I've even seen people use a tall speaker stand placed behind the TV.
Note that theory says the center is better placed above the tv than below because humans are better at localizing sounds below the ear than above. That means that if the center is below the TV in some instances you can detect that the sound is coming from the shelf below the TV as opposed to sounding like it comes directly from the mouths of the actors on the screen.
In practice, I don't think it matters much either way as I have had it both above and below and it sounds good to me either way.