It really saddens me to know that there actually are some people out there, trainers included, who believe that females should not lift anything heavier than a 5lb dumbbell. "It will make me bulky" is something you might here female trainees say. This is one of the biggest myths around and I really wish that it could be buried and hidden away for good.
Muscle is the most important part of the metabolic equation and in order to build muscle, you need to give your body a good enough stimulus. In order to do this, you need to use heavy enough weights to generate enough intensity to provide the needed stimulus. I am not saying that you have to lift extreme amounts of weight and be powerlifters or strongwomen (although you could if you wanted to), I am just saying that you need to stimulate your muscles to grow. Females don't have the genetic makeup to get "big and bulky" as their muscle fiber size is generally smaller than males and they don't have enough testosterone. Building muscle is the real way to get the "tone" that you always hear about. In fact, "tone" is simply having low body fat with muscle showing.
So ladies, don't be afraid to pick up some weight if you really want to get that toned body you always talk about.