Everyone would agree that carrying less weight is a good thing. So no matter whether you are a ultra-light or heavy weight packers, losing un-needed pounds is productive.

Conventional wisdom is that people should carry a backpack that’s <25% of their body weight. This is reasonable for moderate distances but will be fatiguing on a longer hike. There were several experiments evaluating how pack weight impacted soldiers physical performance. A key finding was that on all day exercises, fit/trained soldiers had a measurable increase of fatigue and a drop in agility when they carries more than 10-12% of their lean body weight. This matches my personal experience: I feel no more tired at the end of a 20 miles carrying 17lb as compared to carrying no pack. My one caveat that when over 8 lbs I need to shift some weight to my hips or shoulder pain will be fatiguing through the day. My wife found her sweet spot is <7lb.

There are a number of ways to reduce the weight of your pack. Here are a few ideas which have been very helpful to me:

1. Create a gear list which records items you pack and the weight of each of those items. The two most useful tools for this task are a scale which is accurate to a gram and a spreadsheet or gear calculator (like lighterpack.com) so it is easy to see how your pack’s weight changes as you add and remove items. A decent digital scale can be found at any office supply store or kitchen store. There are plenty of good options for less than $10. If you don’t have a good scale, you can always take your gear to the post office and use their scale.
    
2. Keep track of what you use and don’t use on trips. After each trip remove one or more items that you didn’t use. Over time this will eliminate items which you think you need, but in reality you never touch. For example, nearly everyone I know takes an “emergency blanket”… yet how many people use one? One a day hike, maybe. But when backpacking most people carry a sleeping bag, additional clothing, and a tent, why would you use an emergency blanket.
    
3. Carry only the water you need. I have been amazed to see people carrying 3-4L of water on trails that parallel streams or pass lakes every couple of miles. Take advantage of your water sources. Consider “camelling up”, that is drinking your fill when at a water source rather than just refilling your water containers. Just don’t be reckless. If you are in the desert without water supplies, you might need to carry several gallons :-(. 
    
4. Carry only the food that you need. Don’t bring more than that. This is save weight and simplify cleaning your dishes because you don’t have a small pile of remaining food. Learn how much food you need to stay fueled up. For me, that’s around 18oz / day. Unless you are super lean, you can fuel yourself from your body fat. The normal person can pull around 30 calories / lb of body fat / day. This means that someone who is say 180 lbs, with 25% body fat, could run a 1400 calorie deficit without risking muscle loss. If you are on an extended hike, consider making use of resupply points. In many locations it is possible to resupply with just a moderate adjustment to your route every 3-4 days. Imagine you are hiking a section of the Appalachian Trail for 12 days and you want 2lbs of food for each day. If you make two resupply stops you would start each section carrying 8lbs of food. Alternatively you could start the trip will all the food you need, 24lbs. Each day the food load would drop by 2lbs. You would be carrying more than 8 lbs (the max weight if you used resupply)for eight days. This doesn’t consider other consumables like fuel.
    
5. On trips that don’t require melting snow for water, consider trying a home made alcohol stove, a minimalist pot, and cooking food for that require a modest amount of boiled  water like ramen noodles or couscous. This would cost a few dollars to try, and save a couple pounds compared to the very common Whisperlite Stove + MSR blacklite cook set.
    
6. Carry the right clothing. Wear performance clothing that dries quickly which saves you from having to bring a second set of clothing. Take a small thermometer and keeping track of your comfort -vs- clothing -vs- temperature. You will find this enlightening. In particular I have found that when I am in the midst of a backpacking trip, I tend to require less insulation than I do at home to stay comfortable. Remember you need a lot less insulation when working hard than when sitting around. As you learn your comfort zones you can start to tailor the clothing you bring to the conditions you expect. This often results in having to bring less.
    
7. Remove excess “small items”. Lots of little things add up. I used to carry more band-aids and anti-biotic tubes than I could ever use.  I would also bring a sam’s splint, sling, burn cream, etc. Rather than trying to bring everything focus on life saving (which is typically about technique) and a small amount of  supplies for “day-to-day” issues. Likewise, bring the amount of rope, sun screen, bug juice, etc that you will use on the trip.  Minimize the size, weight, and number of containers. I known people who end up saving more than  pound by reducing the number and size of containers.
    
8. As your budget permits, replace your heavier items with lighter items. People often talk about going after the “big four” (sleeping bag, sleeping pad, shelter, pack) since these items tend to be the heaviest parts of your gear. Unless you are prepared to purchase multiple packs over time, I would suggest being slow to replace your backpack. It really sucks to be using a backpack that doesn’t have an adequate suspension for the weight you are carrying. I have a number of pages which recommend light gear.

Ultralight Twist

I have been strongly influenced by ultra-light (UL) packing style, but I am on the heavier side of ultralight, or maybe the low side of a lightweight style. My gear is mostly ultralight, but I sometimes bring more gear than I actually need to be safe. Hardcore ultra-light folks are willing to give up more comfort than I am to lower their pack weight. A number of examples:

• One set of clothing which is used for hiking, sleeping, etc. If these cloths need to be cleaned, rain gear is worn. If the clothing is wet, body heat is used to accelerate the drying. As a result, a hard core UL will be wearing wet, smelly clothing longer than I might choice to. I do this sometimes, but I often have both hiking clothing and camp clothing. The camp clothing can be layered with my hiking clothing if conditions turn truly cold, but I am carrying more than is absolutely necessary.
• Selecting insulation for safety rather than comfort. For example it is possible to be “safe” though feel chilled. Rather than taking a sleeping bag or an insulated jacket which will keep you feeling warm and cozy in all expected conditions, take insulation which will keep your safe (away from hypothermia and exposure) in the worse conditions you expect, and reasonable comfortable in the average conditions. Select insulation for your body keeping in mind that you have a sleeping bag or quilt for when it will be the coldest and you will be the least active, and will be physical active other times which will lessen the amount of insulation you need. For example, on three season trips I know people who bring just a base + shell because this is all they need when they are active. If the evening is a bit chilly before they are really to go to bed, they use their sleeping bag or quilt as a cape.
• Being a bit thirsty sometimes. You should always carry enough water to be safe… but if there are reliable water supplies, then you don’t have to carry lots of water through the day. Just stop as you need to acquire water. I personally don’t like to stops too often to acquire water, so I carry more water than strictly necessary.
• Push yourself further than you are comfortable. Going UL might mean that you need to push yourselves more than you would on some days so you can get to a resupply, water supply, down below tree line, etc. Often this might mean starting your hike earlier in the day, or hiking later in the evening,

For additional ideas, check out the most excellent book Lighten Up by Don Ladigin with illustrations by Mike Clelland. I have captures a number of other good resources on the web and in the book stores.