I personally think the best golf drill to cure the common slice is the 7 o"clock to 1o"clock golf swing drill. It has helped me and many others to get the ball flying straighter and longer consistently. To make a perfect backswing, you need to maintain a straight posture. Drawing back inside or extending your back at the wrong angle is going to set you up for failure before you even begin. You should also watch the gap between your hands and chest as your shoulders rotate.
At the top of the backswing you must cock your wrists and pause. If you've come this far then your backswing is looking good and you can now turn your attention to your golf swing, which is where most of the problems are created. If you're slicing the ball, you probably have a problem with the first stages of your backswing. That is, you're exiting the proper plane early. Sometimes a bit of physical training may be needed to strengthen your muscles and gain control at the top of the backswing.
Right Angle Drill
Getting great core strength is important as your shoulders start to swing to maintain your swing plane. It is in these unwinding stages that a lot happens and you need to be able to keep your body in position. Don't ever think golf fitness isn't important even if you just play for fun. After all, isn't it more fun to hit fairways than get lost in the trees or the long grass consistently? During every backswing I remind myself to keep my right elbow joined to my hip. I try to exaggerate this as much as I can.
Then, start to implement the 7 o"clock to 1 o"clock golf swing drill by picturing a clock in your mind printed on the ground beneath you. This is one of the great golf tips because it gets your swing planing from the inside to the outside and will in most cases counter the slicing effect on the ball (and can form the foundation of creating a perfect draw for more advanced golfers).
With the imaginary clock face beneath your feet (that is, you're standing in the middle of the dial) target 12 o"clock. As you swing it may feel as though you could be about to hit the ball right of your target, but don't worry about it, your natural rotation will bring the clubface around. A common error that you might make to compensate for the strange feeling is to open your clubface too much. Just give it time and practice and you will be hitting on the proper target line in no time at all.
Also, with the 7 o"clock to 1 o"clock golf swing drill you need to keep your hands parallel. That is, your hands will be placed correctly if your clubface is square. During the swing rotate your right arm over your left to close the clubface. If you've executed the 7 o"clock to 1 o"clock golf swing drill correctly you may feel a bit strange at first hitting the ball straight, or even start to find yourself drawing the ball.
Keep working on it. It is one of the best golf tips available and one that has given a lot of golfers an edge and a big boost to their performance. Keep practicing and learning everyday and your golf is going to improve out of sight in no time.