One of the biggest mistakes I made in the beginning and have seen and heard many others make is simply taking to much stuff. I do not consider myself an ultra lightweight junky, but I dont want to carry excess either. Besides getting decent gear especially for the "big three" (pack, shelter, sleep system) A ton of weight can be saved by simply trimming excess "stuff". Alot of this is trial and error , but searching through forums such as Backpacker Community, Backpacking Light, etc...can give alot of ideas. Rokslide is another good site, although I think it has gone downhill somewhat over the last year I still visit it fairly regularly. Having done the backcountry stuff for over ten years now I still seems like I often get stuck thinking i need to lighten some big money item, when I could often save the same or more weight by simply altering a certain item or leaving it home.
