Great stuff here, including examples of some terrible first lines. Worth reading through for some advice from a master:
Stephen King: There are all sorts of theories and ideas about what constitutes a good opening line. It's tricky thing, and tough to talk about because I don't think conceptually while I work on a first draft -- I just write. To get scientific about it is a little like trying to catch moonbeams in a jar.
But there's one thing I'm sure about. An opening line should invite the reader to begin the story. It should say: Listen. Come in here. You want to know about this.
It's worth looking at. I find myself doing the same, writing down sentences that grab me, for stories I don't even have yet, and jotting them into a notes app (Drafts) on my iphone.
When I'm starting a book, I compose in bed before I go to sleep. I will lie there in the dark and think. I'll try to write a paragraph. An opening paragraph. And over a period of weeks and months and even years, I'll word and reword it until I'm happy with what I've got. If I can get that first paragraph right, I'll know I can do the book.