The cross is a straight punch thrown from the rear hand. With the proper body mechanics, the power of the cross can be greater. Proper technique and body mechanics are as follows: turn the rear foot, knee & hip at the same time, then the shoulder and then the arm, into the punch. Many trainers say that the power of the cross comes from the legs. So it's as if you are pushing off the ball of your foot. And again, do not drop your arm when you are bringing it back.
As with the jab, you should try to throw the cross as straight as possible. Avoid throwing a wide cross, because you will be telegraphing your punch. It too should have a corkscrew motion (refer to The Jab). Your rear shoulder should not go past your lead knee. If you do, you're over-extending your punch, leaving yourself off balance and open for a counter. I would also like to mention that if you throw a cross to the body, be aware that your opponent is going to try to counter you. He may throw a hook, he may throw a downward cross or maybe he may throw a jab to intercept you. Or maybe you'll get lucky; he'll just block the punch with his elbow and not retaliate. Or better still, you'll actually land the punch. Regardless, be sure to bend with the knees and not your back. And always keep your eyes open for a retaliation.