perfect your sprawl as much as possible but to be honest if a good grppler wants too take you down he is going to get you there eventually one way or another im afraid. as for striking his head with your fist as he shoots in for the takedown forget it, you will break your hand trust me. you could try a muai thai style knee but the speed a good grappler can cover distance for a takedown makes that too much of a risk imo as your more off balance. learn more about fighting from your back and pulling into a closed guard for numerous chokes from a good clinch and you will feel more confident. oh and get as many hours rolling with as many different guys as you can so you learn to relax and breath and tire him out first.
try and move with him and anticipate what he is trying to put on you first. trick him into defending himself in one place so you can attack another, for example one of my favourites just now is while you have your attacker in your closed guard grab his right tricep with your left hand and get him in the clinch with your right and pull his head down. usually you will find he will bring his right elbow down on your chest. instantly pop it across and away from you with your now free left hand and go for an arm triangle whilst driving his body away with your legs/hips. you find if his shoulder is high enough you can get extra pressure on his carteroid artery.
or simply get him in a clinch and grab your wrist with your other hand and close his windpipe!