See Xan's Cross Country Backpacking article
1. PACK LIGHT! For longer trips, try to cut down on unnecessary weight. Unnecessary weight that can be avoided:
a. Don't pack cans of food - the cans weigh a lot; pack food already packaged in wrappers, etc.
b. As for liquids, carry enough water on you, attaching a Camelback - or the reservoir - to your pack is a good idea. But other than that - liquids weigh you down. Go with energy bars, not drinks. To restock on water- carry a filter- or purchase an attachable filter for your reservoir.
c. Watch the weather- nights will be cold almost anywhere you go, but if you have warm sleeping bags/mummy bags, you may not need a tent if there will be no moisture. If you do not bring a tent, it is still weight-efficient to pack some light tarps. One can be used to cover your packs, to avoid getting the morning dew all over, and a tarp to put under (or over if unexpected rain occurs) yourselves.
d. Pack just enough clothes. Really think about what you will need (layers, and a fresh pair of clothes or two depending on how long your trip is). Chances are, you might not care about changing clothing a lot.
2. MAKE SURE YOUR ROUTE IS MARKED OUT! If you are not on a designated trail, make sure you have a reliable map or a good GPS system. Just make sure you know the area pretty well and the surrounding escape routes.