The way my distance improved was to teach a guy how to cast that became a distance obsessed person. Then he showed me some things. If you want to do it the way he did it, it was video and reading. That and a ton of practice. There is a ton of good information on a web site in Europe called SexyLoops. It would be a good place to look at. My friend who casts farther than most humans think is possible is also a member here and I hope starts to post here on it. We also have a member here that goes by the name Chuckfluffer that is one of the top distance casters on Earth. His real name is Mike Heritage. WJC is also a serious caster. Pegboy and a couple of others. One of these days we need to get all of these guys on a thread about how to hit the next county with a fly.
I'll just point out a couple of things that will help for distance with a video of Fredrik Hedman. First is tracking. Notice how the rod tip tracks very straight back and forth. Also notice that he drops the rod tip well past 10 and 2. With a longer carry and a longer casts you need to open the stroke up. Notice that he rotates the rod very late in the stroke. You can't see it in the video, but higher line speed means more distance. He gets that with all the previously mentioned details and his double haul, which is also late in the stroke. He keeps the slack out of the line between his hand and the stripper guide. That's more important than it seems. Also, he looks at his backcast. This is something I have a hard time doing thanks to no longer having a good disc left in my neck. If I could add something to my casting that would give me more distance, it would be being able to watch my backcast without a pain in the neck.
By the way, this guy cast a 3 wt. 115'. Not sure if this is the cast he did it, but he is a good example to use. Look at videos of a lot of different guys and experiment. Some things, and by some things I mean tiny details, will work better for you than others. Everybody has their own style in the end. Last, if you look at the guys who win the accuracy contests, it's the same guys that win the distance contests. If you can cast far, you also cast well enough to hit what you aim at.
One last thing. Do video of yourself casting and when you really bang one out of the park look at it. It is also very good for finding flaws in your casting.