Focus on three areas:
Mechanics
Strength
Grip

There are some AWESOME drills here to help you in those areas:
http://www.weplay.com/youth-baseball/drills/skills/throwing

Firstly you need to make sure that you are throwing the ball correctly. Start with the prayer drill with every warm up. Check your mechanics before every throw. Get a coach or some one that knows how to throw correctly to help you fix your mechanics.
Go from prayer drill at 15 feet - 10 reps to standing at 15 feet using the exact same mechanics as the prayer drill - 10 reps, 30 feet - 10 reps, slide step at 60 feet - 10 reps, 90 feet - 10 reps, then 120 feet 10 reps using a crow hop. then beginning to throw the ball at increasing distances. it's not important if you can make the throw on the full, it's important to get the ball to your partner. do this for 10 more reps. This is called long toss, if you are able to do long toss 3 times a week and gradually getting farther and farther apart from your partner, your arm strength will increase very quickly. When you have finished long toss, walk in slowly, throwing the ball untill you are back to 15 feet apart and then throw aroung 10 to 15 reps of fast feet fast hands to warm down.

Avoid long toss on game days. Stick to 90 or 120 feet.

I'd try putting more force into the throw.

get in your throwing position
then you step one foot out
put up behind you
then you a hand up above you then throw

just throw as hard as you possibly can with yuor arm, feet, and hips

Keep your grip on the bat and hit hard. Put force in it

ask for help

talk to your coach