Be careful not to increase the weight to more than you can handle. If you're grunting and contorting your body into weird shapes, you're lifting too much weight, and you're likely to get hurt. Your goal is a controlled movement without any jerking. Otherwise, you'll be relying on momentum instead of muscle. Going slow yields far better results.
Many people make the mistake of holding their breath as if they're underwater or something.
In general, you want to exhale through your mouth as you lift the weight and inhale deeply through your nose as you lower it. Before you do a weight training session, it's important to do at least five minutes of aerobic exercise to get your muscles warm and prepared for the training. Warm-up activities include brisk walking or jogging in place for about five minutes, followed by an appropriate stretching routine. The stretching will improve your ability to move joints through a full range of motion, and in so doing, may help prevent injury. Remember not to bounce when you stretch.