RELAX! You don't need a death grip on the handle bars. Figure out where you are putting a lot of your weight. My guess is, you've locked your elbows and most of your weight is on your hands. Not only will this cause numbness in your hands, but it's a sure sign that your arms will not participate in the suspension of the bike, instead force a lot of unnecessary energy back into the bike say, when you hit ripples in the road, or under panic braking.
1. Don't lock your elbows. You should have a nice relaxed kink in your elbows. Think about it: Is it easier to steer a bike putting pressure on the bars from above, or from a more relaxed angle?
2. Don't grip so hard. Gripping hard is just going to prevent you from reacting well in a panic situation. Learn to be relaxed. You can't be smooth if you're muscling.
3. Don't put your weight on the bars. Guaranteed stoppie if you're in a panic braking situation. Furthermore, you won't allow the front suspension to do it's thing if you're overloading the front by putting your weight there.
Support more of your weight in your back and on your legs. You'll find out just how weak your legs are! But just like someone who downhill skiis. He uses his legs to act sort of like a suspension. Don't feed any unnecessary inputs into your bike preventing it from doing what it would normally want to do: Stabilize itself.