Get a large piece of wood and tightly ratchet strap the TV to the wood so it's not going to budge from it (get this first bit wrong and the results will not be good!) Drill a hole towards the front of the wood and thread a thick bit of rope through the hole. That way you can stand at the top of the stairs and should just be able to slide the thing down each flight of stairs relatively easily.
If the wood is thick enough you may also want to cut a lip in the front of the wood so if it hits the top of a stair it glides over it rather than unbalancing the thing.
On a H&S note, make sure no-one is standing in front of the thing whilst it's coming down though...! And on a paranoid lawsuit note that seems necessary to ensure peace of mind in the crap sue-central culture we live in: If you do the above and hurt yourself, I'm not accepting responsibility - if in any doubt, seek a qualified professional to move said TV.