Weighing yourself to monitor your progress in regards to your nutrition and exercise efforts can be frustrating and leave people feeling helpless in many cases. In addition to weighing I highly recommend taking circumference measurements of the neck, arms, waist, hips and thighs, have your body fat percentage measured with calipers, hydrostatic weighing or a body pod, and finally take pictures of yourself front on, from the sides and from the back in form fitting clothes or a swimsuit. The number on the scale is only a fraction of your actual progress.

Do not depend on the scale as your sole resource for progress, here is why:

Some scales are not accurate, they can be broken or off balance and could read 5-10 pounds lower or higher. If a scale is on carpet, uneven tile or wood it can also appear to be calibrated poorly.

Weigh yourself first thing in the morning. If you decide that you want to weigh yourself daily do it first thing in the morning after you have used the restroom and before you eat anything. This ensures that you will have the least amount of variables that can influence your weight making it less accurate.
Avoid weighing in when you are about to start your menstrual cycle, women retain more fluid right before their menstrual cycle starts. There is a lot of fluid retention that can influence weight gain up to 7-8 pounds. Some women also crave sweets or junk foods right before they start their cycle, often causing a slight increase in weight.

Avoid weighing in when you start a weight-training program. Weight training especially for women can actually cause them to gain weight at the initial start of the program. Muscle weighs more than fat and can develop a lot faster than your body will burn off fat. It is best to rely on measurements as results for your progress over the number on the scale especially when you are lifting weights to lose body fat. It is also important to remember that your body produces more blood flow during workouts, avoid weighing yourself after you workout, this number will also be false.

Consuming excess sodium from fast food, or restaurant foods can cause you to retain more fluid then normal, causing the number on the scale to be higher than normal. This is especially true for fast foods, soups, fried foods, processed meats like bacon, sausage, or cold cuts. Chinese, Japanese, or Thai foods will also cause you to bloat due the high sodium content of the sauces used in those foods.
Do not weigh right after you take a shower or go swimming. Your skin is the largest organ in the body and absorbs fluid easily. After a swim or a shower your body can absorb 1-3 cups of water increasing your true weight by a few pounds.