Hitting, whether in slow-pitch or fast-pitch softball requires the same key fundamentals. The only difference in these two types of softball is the timing of the ball. Like with any sport, practice helps to make someone become better at a specific task. Practice your hitting with different drills with or without a bat in hand. Being a better softball hitter does not necessarily mean hitting further, but strategically hitting the ball so that you may get on base.
Remember your fundamentals. Bend your knees, take a small step with your front foot, pivot your back foot, bring your entire body weight behind the ball, keep your eye on the ball and extend your arms over home plate. If you are making a mistake with your swing or have picked up a bad habit, get rid of it before you become too comfortable with it.
Stand in a batter's box and repeat these motions without a bat. Then repeat these motions with a bat. Practice these motions before every practice to get your body used to them.
Practice your swing. Set up at tee and place a softball on top of it. Make sure the tee is within the strike zone. Swing at the ball on the tee like you would when a ball is being pitched to you. You want to try and hit the center of the softball when swinging at the tee. If you are hitting any part of the tee or are missing the ball completely, make adjustments.
Practice swinging straight at the ball. If you are hitting the softball and it is popping up, you are probably too much on your heels or leaning back too much which is making the ball go upwards. Recenter yourself and keep your weight centered when swinging through the ball.
Swing with your entire body. The majority of the power behind your swing should come from your legs and not your arms.
Hang up a net or even an old bed sheet. Stand bat-length away from the barrier while facing it. Take a swing like you would when in the batter's box. If you hit the barrier at all, your swing is too wide and it needs to be corrected. Your shoulders need to come around before your elbows become unlocked.
Know the timing of the ball. If you're playing slow-pitch, being patient is the key. If you come around too quickly, you will hit nothing but foul balls and pop-ups. If you're playing fast-pitch, you need to have a quick reaction time. Not reacting quickly enough will result in missing the ball.
Have someone pitch to you or go to the batting cages to practice your swing. The more you practice, the better you will become.