We all know the human body is lazy. And while the degree of laziness can vary from person to person, even the most dedicated and well-trained individuals will still fall victim to the body's own energy saving techniques. If you're seeking an adaptive response, that is, if you want to grow, you'll need to actively counteract these tendencies. Here are my thoughts on the subject.
First, let's identify the various techniques your body may employ to avoid extra work. The first and easiest to counteract is what I call the "catch and release" effect. Actually, it's more like the "release and catch" effect, but I like the ring of "catch and release" better :) In essence, your muscles will "drop" the weight during the first part of the negative, or eccentric, phase of the movement, and "catch" it before reaching a fully stretched state. This effect is completely natural even at low poundages, but training past your capacity will most definitely exacerbate the problem. Either way, it's something to watch for and counteract to the best of your ability. I like to employ a "push-pull" mindset when it comes to the eccentric portion of most movements, envisioning the muscle pushing the weight through the stretching phase--rather than simply thinking about "lowering" it.
Another related effect is the body's reliance on connective tissue to absorb much of the stress applied to skeletal muscle. Tendons and ligaments are a lot like rubber bands in that they are very elastic. Connective tissue stretches and contracts, just like skeletal muscle, but in a much more autonomous fashion. While skeletal muscle, in an extended state, has no inherent retractile force, connective tissue does. This means that stretching the connective tissue can elicit a semi-automatic return to statis, with little or no conscious effort on your part. This is especially common in muscle groups where connective tissue can easily assume stress on behalf of the muscle, such as the calves, abs, biceps, and triceps. Try using even shorter ranges of motion from time to time to limit the habit-forming behavior of "bouncing" on your tendons. For example: Performing calf raises or extensions in a manner that significantly limits the stretch of the soleus and gastrocnemius will reduce the natural reliance on the elasticity of the Achilles' tendon to provide momentum throughout the movement, thereby increasing stress on the target muscle(s).
And let's not forget that skeletal muscle is 30% stronger during the eccentric portion of a movement, so actively contracting the muscle against the weight can net you even greater results than you might think. Always do your best to not only achieve a full stretch, but to also achieve a full stretch under tension. Negative sets can be a great addition to a workout, but they should be intended as just that, an addition. For best results, use a weight that you can lower slowly under complete control and make sure your partner only assists during the concentric portion of the movement.