As a rule of thumb, you can figure on using 2 lbs of food per person per day on a 3 season backpacking trip. Depending on your body weight and exertion level, you may need more or less food, but that's a pretty reliable estimation to begin with.
When I pack food for a backpacking trip, I throw a bunch of meals together without weighing each item individually and weigh my entire food bag to see how close it is to 2 lbs per day. If it weighs more, I start substituting more caloric items for less caloric ones until I get it down to the what I'm willing to carry, which averages to about 1.75 lbs per day for a three season trip.
How can I reduce the weight of my daily food intake?
One way to reduce the weight of your food is to remove all of the excess packaging at home. I also rarely carry dehydrated backpacking meals from companies like Mountain House or Backpacker's Pantry which tend to weigh a lot. Instead I break up bulk items like peanuts, chocolates, wheat thins, dried fruits, etc. and bring very caloric foods along like waxed Gouda cheese or big chunks of salami.
I also some pack olive oil in a plastic squeeze bottle and add it to Ramen noodles or soups, but you also get it in little ketchup sized packs of a 1/2 oz each. It has 240 calories per ounce and is a great way to reduce the total weight of your food.
What about Freezer Bag Cooking?
Substituting dehydrated meals that you reconstitute with boiling water, called Freezer Bag Cooking, is an excellent way to reduce the weight of your food bag. It takes a little extra preparation at home to bag everything up in ziplocs at home, but it can be a fun way to enjoy a snazzy breakfast or dinner when you are likely to use your stove.
In all honesty, I rarely bother to package freezer bag meals for weekend trips anymore, although it's very good for longer trips where you have few opportunities for resupply, like Maine's 100 Mile Wilderness.
What if I get hungry?
If you don't pack enough food for a 3-day trip you won't starve to death. Your body should have enough excess fat and protein reserves to get by for a few days. I sometimes pack light in summer, when its very hot, and I know that I won't have an appetite at dinner. I just pig out when I get off the trail to catch up.
How can you avoid bringing extra food on a trip?
When you go on a backpacking trip, you should try to avoid bringing uneaten food home, simply because it's dead weight in your pack. From my perspective, it's better to give it away to another backpacker than to carry it unnecessarily. One thing I do when I'm packing my food bag, is to make sure that the food I bring is exciting and stuff that I know I will enjoy eating. I found out the hard way that I won''t eat boring food when I'm hot and tired, and I used to bring a lot of this extra food home after a trip. Jazz it up. Try bringing hot and spicy foods along, or a special treat: whatever will keep you interested and eating.
Should I bring one or two extra meals in case of an emergency?
I use to bring extra food along for this reason, but don't do it anymore. I never used it and it was a pain to carry the extra weight. I figure in an emergency, I can get into my sleeping and drink hot tea to stay warm. I also have a few extra pounds on my love handles that I can burn off.
I should add that I hike with a SPOT satellite personal locator beacon which I could activate in an absolute emergency if I couldn't survive without assistance. It also lets me send a message saying I'm overdue but ok, which is good in cases where I'm running late on my route, but intact.
If you have any more questions on this topic, please leave a comment below and I'll be happy to answer them.