Back when I was first trying to figure out what I needed to be eating on a daily basis, I wish someone had been able to explain it to me a little better... now that I understand it, I hope someone else who doesn't have the benefit of a software program to calculate it finds this useful.
The formula to find your Basal Metabolic Rate which is the number BEFORE you account for lifestyle and activity is:
WOMEN: 655 + 4.36 X Weight (lbs) + 4.32 X Height (inches) – 4.7 X Age = BMR
MEN: 66 + 6.22 X Weight (lbs) + 12.7 X Height (inches) – 6.8 X Age = BMR
As an example, here's mine:
655 + (4.36 x 127) + (4.32 x 62) - (4.7 x 30) = 1336
This is NOT the number that you should be subtracting calories and exercise from! This is the barest bare minimum that your body needs to keep itself going!
You then take this number and add your "occupational allowance", which is an additional percentage of your BMR that takes into account what you'd typically expend going about your day.
Quiet sitting = BMR +30%
Office work (light activity) = BMR +50%
Housework (moderate activity) = BMR +70%
Heavy manual labour (i.e. construction, landscaping) = BMR +100%
Mine, honestly, changes from day to day. On days that I do auction work, I fall into the "office work" category, and my daily calorie needs would be 2004. Other days, I'm pretty well sedentary, and so it would be more like 1737.
That's STILL not necessarily the number you subtract calories from when you're trying to lose weight!
You also have to add in any daily exercise... and extra activity counts. I don't vacuum every day, so on days that I do, you'd better believe I count it. Same for shoveling snow, grocery shopping, and anything else that involves expenditure of energy. An excellent site for figuring out how many calories you're burning with a specific activity is http://www.caloriesperhour.com/index_burn.html (thanks Shell!)... I like this one because it takes into account your age, weight, height, and gender - all of those things do affect how you burn calories.
Once you've added all of that together, THEN you can start thinking about decreasing calories.
Interestingly, the site that I got this information from recommends decreasing calories by only 350 a day (from that total number, including exercise), resulting in a weight loss of 1/2 lb a week, because it's a slow, gradual loss that won't sacrifice muscle.
Like has been said before, I think, if you're under the supervision of a doctor with your weight loss program, and they specifically want you eating low calories for some reason, that's one thing. But I definitely agree with what's been said already about low-calorie diets being counterproductive. Not only can it kick your body into starvation mode, but mentally and emotionally, it's very difficult... when I eat less than 1400 calories, I feel like a slug and I can hardly move. (And it's no wonder - my body isn't getting what it needs!) When you get to that point, you're just setting yourself up for binging, overeating, or eating stuff you shouldn't be eating. It just makes the entire process more difficult than it honestly needs to be. Trust me, I've been there. 
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About BMR... somewhat related
