One of the most common causes for a sliced shot is an upright swing. This means the swing is too vertical. From this position getting the clubface square at impact is quite impossible. More often than not, it stays open, leading to the slice. This is a very common swing fault for juniors, women, seniors or players with reduced power or mobility.
The best part about this drill is its simplicity. First, find a side-hill lie that you can hit balls above your feet from. Second, take some practice swings to get a feel for your shot. Finally hit golf balls. The simple fact that your ball is positioned above your feet will force your swing path to be flatter. This alone will help to diminish your slice!
Give it a try!