Once you start slicing your returns, it is difficult to break out of the habit for at least the remainder of the set. You need to go for your returns right off the bat and at least see if you can hit through the ball. The slice return is always going to be there as a backup if you need it.
Do you play doubles? The reason I ask is that it is much more difficult to get away with a slice return and it forces you to come through your returns.