If you have air leakage through the sides of your mouth while you play then you probably "smile" when you play, which is pulling the corners of your lips back. This allows air to escape through a thin slit between your lips. To fix this you should play long tones and focus on pushing the corners of your lips forward while you play.
Say "Oooooo" outloud and remember how the sides/corners of your lips are formed. That's how they should be positioned while you are playing to get rid of the leaky lip problem. Place the mouthpiece alone inside your mouth, your teeth on top and your lower lip underneath the reed as usual, then let the sides of your lips move forward as if you were saying "Oooooo," if done properly your lips will form around the mouthpiece and block any air from leaking.