Not many things on the cricket pitch top hitting the ball cleanly over the boundary. It looks and feels great. To improve this skill you need to have good technique and work on your 'speed-strength' during the winter. Speed-strength, in shot making terms, is how fast you can swing the bat. The faster you can do this with good technique the harder and longer you will be able to hit a cricket ball. This velocity is developed in three distinct phases:

1. General training. Here you build a base of strength with a range of compound weightlifting style exercises such as the bench press and chin ups. Most people can only do general training in a gym as bodyweight training tends not to be enough for this kind of strength development. New trainees can see a significant increase in bat velocity purely from this method. Make sure your programme is designed by an expert strength development for sport.
2. Special training. Once a base of strength is built up you can move onto more power specific exercises such as cleans and medicine ball throws. Hitting a ball long requires your muscles to generate explosive, rotational power through your whole body so this training is teaching your body how to deal with such movements. As your body adapts to these exercises you see a progressive improvement in bat speed with it. Again, speak to a sport strength specialist to get a programme that is suitable for cricketers.
3. Specific training. More advanced players can finally move onto training with overweight and underweight bats. Lighter and heavier bats can be used in the net to play shots. The heavier bat develops strength, the lighter bat speed. Be careful with this approach as if you go too heavy you lose technique as you try to adapt to the weight. A few ounces difference should be fine either way. Once you have batted with the different bats, finish up with a bat that is your usual weight to counterbalance.

Beginners can stick to general training and still improve greatly, but once you have a base of strength you want to develop seriously you can periodise the training to rotate between all three methods to get the best bang for your buck.