Jerry,
The balsamic vinegar works a treat, once cooked, it gives an almost caramel sweetness to the steak. I use enough to coat the steak that I am using and just a little more, as the steak tends to soak it up quite a bit. But as I said, I do use rib-eye rather than sirloin, so that may differ slightly.
I think the key to getting a good finished product is making sure you have bought a good steak in the first place. Also, make sure it has time to breath out of any packaging, before being cooked. There is nothing worse than cooking a supermarket steak straight from it's plastic tray.