Tips

Re-package items into smaller, lighter containers
Many items like food, bug repellent, and sun screen are packaged in much larger volume containers than you need for the length of backpacking trip you are taking. Estimate how much of each product you need for the trip, and re-package it in a smaller container that holds that amount.
Wear a wind shirt.
Wind shirts from GoLite, Montbell, Montane, and Patagonia now weigh less than 3 ounces. The addition of a wind shirt to your clothing system can add tremendous comfort and significant warmth, allowing you to wear lighter base layers, lighter rain shells, and lighter insulating garments during active exercise in cold conditions. The bottom line: a wind shirt extends the comfort range of your clothing system, and allows the other pieces to be lighter.
Get a lightweight down jacket
Don't pack around that ultra heavy old down jacket. Lightweight down jackets from Montbell and Patagonia weigh less than 8 ounces and provide a great deal of warmth. A clothing system for lightweight backpacking consists of base layer, mid layer, and outer layer. Down has the highest warmth to weight ratio and is very compressible. Lightweight down jackets compress to the size of a small grapefruit.
During cold nights, sleep with your lightweight down jacket on
It doesn't make much sense to pack around an insulated jacket, and a sleeping bag that is rated for temperatures well below what you are expecting. Instead, select a sleeping bag that is rated for the expected temperatures(or even above the expected temperature) and extend the comfortable range of the sleeping bag by sleeping with a lightweight insulated jacket on. Lightweight down jackets from Montbell and Patagonia weigh less than 8 ounces and provide a great deal of warmth. A clothing system for lightweight backpacking consists of base layer, mid layer, and outer layer. Down has the highest warmth to weight ratio of insulating materials and is very compressible. Lightweight down jackets compress to the size of a small grapefruit.
Develop your skills
Attending backpacking clinics, wilderness medicine courses, wilderness survival programs, and of course, actually getting out there and logging some trail miles provides the best foundation for reducing your pack weight.
an introduction to lightweight backpacking
