Lightweight backpacking is a process where you learn to prioritize and simplify your gear by trimming unneeded weight off of your existing gear, by choosing new gear that weighs less, and by "roughing it" according to your personal preferences by leaving heavy luxuries at home. Lightweight backpacking lets you carry less weight which in turn allows you to travel lighter and faster and can allow you to minimize impact on the outdoor environment.
Lightweight Backpacking Tips
Plan, plan, plan. Know what weather, terrain, etc. your trip will encounter and pack accordingly. Rushing around during last minute packing for a trip will have you tossing things you probably won't need into your pack.
Pack multi-function gear that can replace other items. Duct tape (repair kit, first aid), bandanas (wash cloth, towel, bug swatter, headgear), trek poles (tarp poles, tripod, bear killer).
Carry less water if possible. Water is heavy. Get fully hydrated at the trailhead and at reliable water sources along the trail. Carry water only along dry stretches.
Learn to live in a smaller tent or even under a tarp. It took me a few trips to learn to love a small 1 person lightweight backpacking tent after lugging around a roomy 2.5 person dome around for years.
Repackage your food at home to eliminate unnecessary heavy packaging and reduce trash you will have to pack out. Every ounce counts.
Plan gear choices with your backpacking partner(s) to share weight and reduce unnecessary duplication of heavy items.
Weigh every piece of your gear with a postal type scale and put the data into a spreadsheet so you can see exactly where all your backpack weight is coming from. Then you can see where you can shave off ounces or even pounds to have a lighter backpack.
Keep track of any backpacking gear that wasn't used on your trips. Use this information to help decide whether or not to bring those items on your next trip.
Mix it up, experiment, and challenge yourself. Try leaving your favorite heavy luxuries behind on some trips... leave your thermarest, camera, GPS, and even your tent home on some trips.
"The most important passion of life is the overpowering desire to escape periodically from the strangling clutch of mechanistic civilization. To us the enjoyment of solitude, complete independence, and the beauty of undefiled panoramas is absolutely essential to happiness."