The ability to throw effective punches is up there with the most important aspects of combat martial arts and sports.
A common mistake when sparring / fighting is the failure to keep your feet directly under your punches. Overreaching puts you off balance and isn't conducive to delivering quality high impact strikes.
Having good balance increases your punching power, allows for better counters, and gives your greater defensive abilities.
Overextended techniques leave too much hang time at the end of the punch, which leaves you unable to follow up and leaves you open to potential counter attack.
In order to address this problem, you need to ensure you throw techniques whilst maintaining your balance. There are several basic pointers to help you do this:
Move your centre of gravity: When you strike, move your centre of gravity into it, don't let your upper body swing on its own.
Bend your knees: Keep your knees bent in your stance to help keep your centre of gravity with you when moving and throwing techniques.
Ground your feet: Keep your feet on the ground when you throw punches to help keep your balance and increase power.
Punch Lighter: You don't always have to punch your hardest. You can increase your recovery time and improve your defence by throwing lighter punches.