You need to keep the flex plate from moving, theres a little plastic cover over it (like bio said between the trans. and motor) I've always done it with a pair of vicegrips, works great.
Also, you're gonna need quite a bit of leverage if you're doing it by hand. When I did mine i used a 1/2 breaker bar that's 3ft long, with a 2ft extension on that, and it was still a *****.