I stick to the following rules regarding workouts:

Train Hard - you get out of a workout what you put into it.  Don't just go to the gym and go through the motions.  Research and devise an effective workout routine.

Keep the Body Guessing - do not do the same routine every week - switch it up a bit.  There are many variables to change throughout the week to keep the body guessing which include: rest days (unless your schedule is impossible to work with - mix up what days are rest days), exercises (different exercises workout the same muscle groups), sets (giant, super, or drop sets), and reps. 

Be consistent - If you have 2-3 rest days a week and workout 4-5 days a week then keep it that way.  Don't cheat yourself.  If you have a busy schedule you still NEED time for workouts.

Recover - Do not neglect rest days.  It is almost impossible to put into words how important rest days are for your overall transformation.  Rest and recovery are absolutely imperative.

Find Motivation - Motivation comes in many forms and is often neglected once the soreness of a workout kicks in or the initial excitement wears off.  Maintain your motivation and keep moving forward.

Here is a one week example workout on how to train hard, keep the body guessing, recover, keep it consistent, all the while maintaining motivation:

Monday - Legs

Morning cardio - 20 minutes

Leg Press - 3 sets, 12-20 reps

Hamstring Curl - 3 sets, 12-15 reps

Leg Extensions - 2 sets, 15-20 reps

Evening cardio - 20 minutes

Tuesday - Chest/Tricep

Morning cardio - 20 minutes

Flat Dumbbell Press - 2 sets, 6-10 reps

Incline Flyes - 2 sets, 8-15 reps

Straight-arm Pullover - 2 sets, 10-12 reps

Rope Extensions - 3 sets, 10-12 reps

Lying Cable Extensions - 3 sets, 10-12 reps

Overhead Dumbbell Extensions - 3 sets, 10-12 reps

Evening cardio - 20 minutes

Wednesday - REST!

Morning cardio - 20 minutes

Evening cardio - 20 minutes

Thursday - Back/Biceps

Morning cardio - 20 minutes

Reverse-grip Pull Down - 3 sets, 8-10 reps

Bent Over Barbell Rows - 3 sets, 10-12 reps

Hyper extensions - 2 sets, 10-12 reps (use a weight)

Straight-arm Pull Down - 7 sets, 10-15 reps

Concentration Curls - 3 sets, 12-15 reps

Dumbbell Alternate Bicep Curl - 3 sets, 12-15 reps

Cable Curls - 3 sets, 15-20 reps

Evening cardio - 20 minutes

Friday - Shoulders/Calves/Abs

Morning cardio - 20 minutes

Military Press - 3 sets, 6-8 reps

Side Raises - 3 sets, 12-15 reps

Upright Rows - 3 sets,  7-10 reps

Rear Deltoid Raises - 7 sets, 12-15 reps

Weighted Sit Ups - 3 sets, 15-20 reps

Seated Calf Raises - 3 sets, 18-20 reps

Evening cardio - 20 minutes

Saturday - REST

Morning cardio - 20 minutes

Evening cardio - 20 minutes

Sunday - Legs

Morning cardio - 20 minutes

Leg Extensions - 2 sets, 30 reps

Hamstring Curl - 2 sets, 20 reps

Walking Lunges - 3 sets, 40 reps

Standing Hamstring Curl - 3 sets, 15-20 reps

Leg Press - 3 sets, 50 reps

Repeat Exercises 1 & 2 - 18-20 reps each

Evening cardio - 20 minutes

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Cardio can be stairs, walking, stationary bike, or other cardio machines/activities.  My workout from the previous day typically determines what cardio exercise I do the following day.  For example, if I do a leg workout then my cardio the following day is more than likely not stairs.  Regardless of how sore I am (do not confuse with pain/injury) from a leg workout I will still do my cardio, it will just be something easier on my legs like a stationary bike or a brisk walk.