That card is already clocked at 900MHz. Lots of factory-overclocked 560 Ti cards running at 950-1000MHz are having trouble with overheating and maintaining their overclocks. Users are having to down-clock the GPU to get stable in games.
You can try increasing the GPU core voltage by a small amount and then going for 925MHz or so, but you won't likely get stable at 950MHz or higher.
The cards that are having problems at 950-1000MHz already have non-reference dual-fan coolers with heat pipes and such. That likely won't help.
Like I said, try to overclock it a bit further than it already is. But you likely won't get 1000MHz without a fairly large voltage increase and water cooling. The 560 Ti GPU just wasn't designed to go that speed.