Problem: Your Body Is Fighting Back
If you've tried numerous diets and your results are weight gain or instant plateaus you need to listen to your body. It's trying to tell you that your metabolism is not functioning properly and that weightloss is impossible under the conditions that you have created.
Solution: Stop dieting or starving yourself
Your metabolism functions at it's best when you eat adequate calories, when you eat smaller/balanced meals more frequently, and when you are not indulging in excessive amounts of sugar, fat, alcohol, and carbs.
Helpful Guides:
The 3-Hour Diet (TM) : How Low-Carb Diets Make You Fat and Timing Makes You Thin
Curves

Problem: You Don't Workout
Research study after research study confirms that permanent weightloss is only acheived when you eat right and exercise regularly. Relying only on your diet makes it tougher to lose and it makes the process incredibly slow. When you combine healthy eating with great workouts you see changes much faster.
Solution: Start With Easy Workouts
It doesn't matter if you have 10 pounds or 100 pounds to lose, physical activity should be a part of your plan from the get go if you want to get lasting results. Just 15 minutes a day of cardio strength training and walking can make a huge difference.
Helpful Guide:
Quick Fit : The Complete 15-Minute No-Sweat Workout

Problem: You Lack Workout Intensity
Maybe you do workout but you never break a sweat or push past your comfort zone. After a while your body needs more intensity to continue losing and to prevent gaining.
Solution: Super-Charge Your Workouts
Learn how to up your intensity by buying a tougher workout dvd, hiring a trainer, or workint out with a friend who is in better shape than you. You'll quickly discover how much you're cheating yourself and what changes you need to make.
Helpful Guide:
The Firm - Total Body - Maximum Body Shaping

Problem: Your Life Is Out of Balance
70 hour work weeks, packed social life, stressful family life, and other demanding responsibilities drain your energy, create stress, and affect your sleep and hormones. Live under these conditions long enough and you're body will become so unbalanced that you won't be able to lose weight.
Solution: Prioritize Your Life
You're just as important (possibley more important) as all the people in your life. You need to schedule time for your workouts, your meals, your sleep, and your relaxation in order to bring your life back into balance so that your body in turn becomes balanced and primed for weightloss.
Helpful Guide:
Body for Life for Women: A Woman's Plan for Physical and Mental Transformation