Been hacking at this for the last couple of months, due to a bad experience on my first trip to the mountains. I discovered that my 30 lb FSO weight was not going to let me cover ground like I wanted to. So, I started working on both my waist and my pack seriously.
The four biggest tips I can offer to anyone who wants to drop weight are:
1.) Don't carry things you don't use.
With the exception of your first aid kit (which shouldn't contain more items than you know how to use), don't carry anything that you wouldn't use under normal trail conditions. Example: if you're going to be hiking (or biking, or...) long distances solo, don't carry a second tarp. Your poncho (or other rain gear, whatever) will do for when you're cooking dinner, if you know how to set up your stove in the rain (which, honestly, you should know how to do if you're going to be out there alone, as a safety issue, anyway).
2.) Get a scale and weigh everything.
Knowing your gear weights is the second-most important thing you can do. Once you do know them, you can start making a good run towards finding lighter gear.
3.) Think about your gear as a system.
Once you start thinking about your gear as an intricate system, in which certain items support one another, instead of thinking about it as stand-alone gear, you can make certain compromises that allow for lighter gear. Example: instead of using an heavy, large tarp to keep you dry on long solo hikes when you're in your hammock, you can use a smaller tarp and an undercover that doubles as a poncho, thus losing weight. Another example: instead of bringing a base layer, fleece layer, puffy layer, wind shell, top quilt, and rain gear, bring a base layer, fleece layer, a top quilt that doubles as a puffy layer, and your rain gear.
4.) Substitute experience for gear.
It's essential to have experience out on the trail. You never know what you'll wind up running into out there, and the single greatest survival tool you can ever carry sits right between your ears. Over and above that, though, is the fact that you can sub experience for heavy gear. If you know how to use a wood stove even in soaking conditions, you can delete the weight of fuel from your pack. If you know how to take advantage of terrain features to cut wind, you can carry less insulation and rain coverage. Etc., etc., etc.
Those are the four greatest insights I've had so far as to lightening my pack. The trick, however, is to ensure that you are still safe while doing this. I know my limits; know yours before you go out there.
Before you decide to go out and purchase gear, think about the sorts of trips you want to make. If conditions are going to be above freezing, don't bring a zero-degree bag; bring a twenty-degree or even a thirty-two-degree bag (but, if you go the latter route, make sure you know how to make a survival fire!). If you're going to be in the desert, make sure you bring enough water (including the storage equipment for it). Etc.
Backyard (or car camp) testing is essential before you hit the trail. Short-mileage overnighters are also a wonderful way to get your feet wet before winding up thirty miles from the nearest road with freezing conditions and no way to make a fire.
Hope it helps!