Don't Lift Hard - Lift Smart

Make sure your workout is aimed at your goal. Usually we can find that we are not lifting the right way for the results we are trying to achieve. By analyzing our workout and adjusting it frequently to make sure it meets our goal is the sure way to make the kind of gains/definition/losses that you are intending to get.

If you are lifting for Size/Power:
Ideally: 6-8 reps & 2-3 sets with some time in between sets (more then a minute, less then 5), and some time between exercises
You want to do High Weight, Low Reps
This means using weights for your exercises that you cannot lift more then 8 times.
If you can lift a given weight more then 8 times then you need to increase weight. By keeping the reps below 8 and the weight high you are forcing your body to use it's fast twitch muscles - this is how you gain size and power.

If you are lifting for Definition/Endurance:
Ideally: 12 - 15 reps & 4 sets with only alittle (a minute or less) time in between sets and alittle time in between exercises
You want to do Low Weight, High Reps
This means using weights for your exercises that you can lift atleast 10 times, 12 is better By keeping your reps high and your weight low, you are forcing your body to use it's slow twitch muscles - this is how you gain muscle definition and endurance.

On any lifting schedule also make sure that you are operating at a caloric surplus everyday and make sure you eat enough protein. Roughly 1 gram of protein for every pound of body weight is a good equation. Remember that no matter how much you lift, if you do not intake enough protein or calories then your body cannot make more muscle.