Some one else recently said something that really rang true for me.
The most important weights in the weight room are the tiny little 2.5lbs plates. I try to move up at least 5lbs every week on my barbell exercises, which means those little suckers are getting a lot of use.
The way you stimulate your body to grow is to continuously stress it with heavier and heavier weight, and then allow it time to recover and adapt. I work each of my major muscle groups no more than once a week, so by the time I get back around to working them again, they have fully recovered and adapted to bear new heavier loads.
This is under ideal circumstances, of course. Sometimes I don't eat properly throughout the week or I don't get enough sleep the morning before my workout or something and it compromises my goals, but 5lbs a week is definitely doable.