First of all, always lift using your legs, not your back. And now, it is time to strengthen your core muscles. Lying down, do leg lifts (lift your legs to 45 degrees, hold five seconds, lower, do that ten times). Now, do planks. Planks facing down, planks on your right side, your left side, and inverse, each for 20 seconds to start. Do it everyday, and after a week, increase to thirty seconds a side. Keep going until you can do each plank for five minutes.