An easy way to increase the eccentric (negative) loading is to do the concentric or lifting part of the rep with both legs, then the lowering part with one leg. After you hit failure with the single-leg eccentrics, continue the set by using both legs and a controlled negative.
Alternate legs on each rep. Make sure to do the eccentric portion in a controlled fashion to take advantage of the overloading effect. Don't just let your leg freefall when lowering the weight. Do 4 sets of 8 reps.
Maybe you're not injured and you don't plan on being injured. (Ha!) Regardless, this article contains secrets to the weightlifting universe that will help you understand how the body works and, as they say, knowledge is power.