Is the reason for the slippage due to the grip size not being what you measure for, or is it due to sweaty, slippery hands?
If you feel it's because of the grip size, then I'm curious to know why you are playing with a racquet two sizes too small for you
If it's because of sweaty hands...wow. You must be absolutely dripping! If that's the case, the only thing I could suggest would be to keep a good supply of dry towels on hand, and make sure you're constantly drying your hands and the handle during the changeovers (or if you're not playing a serious set, but just hitting around, then make sure you just dry your hands often).
A third idea (just brainstorming for you here) is that you could experiment with some of that "tacky" stuff that NFL receivers put on their gloves...maybe if you put some of that on your racquet it would help.
However, I'm inclined to believe that if you've had the racquet fly out three times, you're either (a) not gripping the racquet properly, (b) not gripping the racquet hard enough, or (c) not paying enough attention to gripping the racquet. Either way, it should be something you can fix without resorting to a wrist strap (which, as has been pointed out, is not allowed by the rules and would get you some strange looks on the court--i.e., "what kind of player needs a strap for his racquet??")