1. If you want to be lean, sexy, and hard, you should train with weights.
A desirable female physique is one that can only be achieved by moving some serious iron in the weight room! But what about all the talk about weight training making women big and bulky? First, it is physiologically impossible for you as a woman to put on large amounts of muscle mass; you're body's hormonal makeup is not one that will allow you to do so. God never intended for women to look like men (go figure), so he made the chemistry of each gender's respective bodies different. Regardless of how you train, how often you train, how much protein you eat, etc, you're not going to even come close to the big, bulky physique of a female bodybuilder. Science tells us this, not me.
2. If you want to be lean, sexy, and hard, you should train HEAVY.
Yeah, I know what they told you, lighten the load and go for the burn-- bull sh#@. To comprehend why this is indeed nonsense, we have to understand a few things about muscle tone in general. There are two types of muscle tone; myogenic and neurogenic. Don’t get thrown off by the science; the first simply refers to your muscle tone at rest. It is affected by the density of your muscles; the greater the density of your muscles, the harder and firmer you will appear. Heavy training increases your myogenic tone through the hypertrophy (growth) of the contractile proteins myosin and actin (myosin and actin are by far the most dense components of skeletal muscle). Training in higher rep ranges promotes more sarcoplasmic (fluid) hypertrophy, as does steady state cardio (treadmill) which in turn yields a "softer" pumped look. If you want to be hard, firm, tight, etc, the latter is certainly not the way to go
3. If you want to be lean, sexy, and hard, you should train with compound, multi-joint movements.
Forget the butt blaster, forget leg extensions and leg curls, and forget "muscle sculpting" with Susie the hyperactive personal trainer. If you want to sport a hard body, you better start training heavy. Big, compound movements such as the deadlift and the squat are superior to machine, isolation-type movements for hardening up your thighs and butt as they allow you to use maximal weight while training a number of muscle groups simultaneously. Another benefit obtained by performing multi-joint compound movements is increased confidence. With strength comes confidence.
In Closing, Remember this!
It's not important that you move big weights; what is important is that you are selecting and lifting loads that are heavy for you. Over time, you will get stronger and the poundage you can handle will increase. So, for you as a female trainee, a "heavy" load can be defined as a weight that you can lift in good form for 3-6 repetitions.