[s]One option to save a little weight, at a low cost, could be to bounce your rain fly ahead of you and take it only when there is rain in the forecast. If you look at the weather in town, and you have 0% chance of precipitation for the next 4 days, bounce it to the next town.[/s] (Probably a bad idea for a thru-hike.)
If you're willing to spend a little bit of money and do some research, I'd look into getting a "tarp + bivy/bugnet" or a "tarp tent" setup. You can find light (but expensive) options for both of these setups through "cottage companies", such as ZPacks.
A tarp tent is pretty much like a normal tent but it is made out of very lightweight material and are (typically) single-walled.
A tarp + bivy/bugnet combo consists usually of a tarp, groundsheet, and some kind of bivy or bugnet for protection. Some don't use a bivy and simply use a standard headnet at night for bug protection. This is the most versatile setup (imo) because you can cowboy camp with just the groundsheet when it's nice, and pitch the tarp when the weather gets bad. You can also get the entire system to come in at around 1 lb - 2 lbs. But, it also requires the most skill and experience to get right. If you do go this route, make sure to do a few shakedown hikes before leaving.
Both options (typically) use your hiking poles instead of tent poles.
Here are zpacks tarps, and tarp tents:
A good article on transitioning from a tent to a tarp setup:
http://sectionhiker.com/how-do-you-slee ... er-a-tarp/
I'm currently transitioning to a tarp setup, so I don't have personal experience with it, but this is basically what I gather from the research I've done.