The funny thing is, the first line is usually one of the last things I come up with. I think it's for several reasons. The first being that I may not have started my story in the exact right place. In fact, I almost never do. My writing usually starts out as some kind of summary and I have to come back and beat it into submission later. Yes, I'm violent with my edits. I use heavy machinery like chainsaws and get ruthless cutting every possible thing I can without jeopardizing the understanding of the story.
The second reason the first line is so difficult and comes toward the end of the process is because sometimes to find the exact right way to start my story, I need to see it framed in it's entirety. I need a completed draft to guide me to the exact right beginning. I know it sounds silly but it's kind of similar to how you can't put the cap on the soda and send it out to the store until the liquid is inside. If you try to cap your story too soon, you could end up with an empty bottle, or the wrong size cap i.e. the wrong opening line. But, if you know where your character is at the beginning and then how they change at the end, then you have a better chance of finding the right opening line.
Even still, it usually takes a huge stroke of inspirational genius to pull the right first line out of my ass. Yes I said it, my ass. I never know where the good first lines come from or when they are going to arrive but they show up when they are good and ready and I have zero control over it. In fact, I'm waiting for the perfect opening line right now for my WIP. (You hear that Orion??? **hint hint nudge nudge**) And it's always a giant source of frustration for me until the exact right line comes to me.
I wish there was some trick to finding the right first line. Maybe there is and I just haven't found it yet. If you figure it out let me know.
So how do all of you get the right first line and at what point of the process does it arrive for you? Any tips or tricks to share?