Steve is absolutely right.
Here is the most common pitfall that you might encounter. When you enter into a massive weight loss program, you are probably really psyched up. In earnest, to avoid muscle loss, you start lifting weights everyday. This is definitely NOT what you want to do.
Especially when your caloric intake is lowered into the deficit range, your body is less likely to be in an anabolic state (a muscle-building state). You need to stimulate your muscles for them to grow, but realize that the actual act of exercise destroys your muscles. It is just the rest period after an exercise session where they grow.
Do you see the balance that has to be made here? You need to exercise and stimulate your muscles without overdoing it. Otherwise your overuse of the muscle will not be compensated for by muscle growth. Especially on a lower calorie eating plan.
Two days on/one day off, two on/two off, one on/one off, whatever fits your schedule.