Feel the negative, slow on the way out, lower under control; these are all phrases that are common to take advantage of the eccentric portion of the lift. Let me break down why this is important and how I do it.
Eccentric is a lengthening of the muscle fibers (concentric is the contraction). When you perform a movement, one half is concentric, the other is the eccentric. It's this second part that not only often gets overlooked, but really is a big bang for the buck.
The eccentric portion can cause a greater stimulus during training, which can result in more muscle growth and/or strength. You are also stronger during this phase of a movement.
For both of the reasons above, you want to slow down and take advantage of this portion. First to get more training stimulus and the second is to make it more difficult.
The method I like to do is to actually try to resist the weight as the muscle lengthens. By doing so, you keep the tension on the muscle plus slow down the lift. An advanced technique is called negatives, which occurs when you can no longer do the concentric portion, but have assistance on that portion and then lower the weight on your own. Be careful as these put a lot of stress on the muscle and should be used sparingly.
Next time you're in the gym, check yourself and see if you're really getting all of the positives of the negatives.