The X-Grip is (of course) based on the principle that anything that's bottom-heavy tend to be stable when held from above; and -- due to momentum -- heavy things help dampen high-frequency shakes.
1) You might want to make sure your setup is bottom heavy. With a light DSLR like the D7000, you might need to add weights to the bottom of the grip. I've seen people attach a few pounds of wrist / ankle weights which can be bought from any sporting goods store.
2) Ease up on the grip, as well as your wrist & elbow, as much as you can. The stability of the system is due to the bottom-heavy setup, not how tight/stiff your grip is. By gripping tighter you're only transmitting more shake to the system (as you've already discovered).
The *downside* to a bottom-heavy setup is the tendency for the system to pendulum and rock side-to-side / front-to-back. So you need to find the right compromise -- too little weight and your videos come out shaky; too much weight you'll really notice the rocking effect.
If you have a really hard time keeping a light grip, then you might consider making or buying a system like the Glidecam. Glidecam works on the same bottom-heavy principle but it has a pivot to isolate your movements, so it doesn't matter how hard you grip it.
Personally I use a Steadicam Merlin which is a much more refined solution than the Glidecam, combining a very precise way to control weight distribution with a high quality gimbal to effectively isolate angular movements. You can get beautiful results but the downside here is $$$.
In any case it takes a ton (and I mean, a ton!) of practice to effectively use any of the above systems!