Shooting a crossbow consistently takes practice and attention-to-detail. Your grip plays an important role in shooting accurately. With a bit of practice, you will improve your groups and increase your effective range.
Crossbows come in varying shapes and sizes. Most crossbows today feature some type of rear pistol grip. These designs, however aesthetically pleasing, lead to grabbing. That is, throttling a grip much like one holds a hammer or hatchet. This results in excess torque as you squeeze too tightly.
Instead, center the base of your thumb on the back of the pistol grip. By placing the thumb base (also called the heel) on the centerline of the grip, you are at a mechanical disadvantage; unable to grab the grip on release. In addition, you should relax the grip, using it as more of a support than a rigid truss.
Your front or post hand, contributes greatly to shooting accuracy. The post hand is the foundation for the forward section of the crossbow. In fact, the less control you have on the forward portion of the bow, the more errant your shots will be. When thinking of the forward hand, think of it as a pillar or foundation of the crossbow.
For those searching for a better post, try this. Lay the forearm on an open, upward facing relaxed palm. Point your middle finger toward the target; by doing so, you consciously think about relaxing the hand. Avoid grabbing the forearm, as this rolls the limbs and pulls the shot.