Trouble at the End

I have a problem.  I'm nearing the end of the first draft of my novel, but I'm not exactly sure how to end it. 

My story has become a trilogy in my head, which complicates my writing a bit.  I have an outline of the high points of each book, but I have to find the happy balance between keeping the first book as a stand-alone novel so that it will actually be publishable, and making sure that the second book is appropriately set up to continue the story without becoming boring filler as so many second-in-a-trilogy books tend to be.

So this brings up a good question: what makes a satisfying and complete ending?  When you, the author, know that there will be more than one book and know what's going to happen next, how do you keep the story self-contained, while still leaving the right number of loose-ends to bring people back for the next installment?

One of my first posts was about the book Ariel, by Steve Boyett.  I hated the ending of that novel.  Now, I don't know if he originally intended it to be a multi-book series (a sequel was published just last year, though the original was published in 1983), but regardless, the ending was dissatisfying and now I have no desire to read the sequel.  I want to avoid that if at all possible with my own readers.

I also recently read the first two books in a trilogy by Celine Kiernan, and I thought the ending of her first book was wonderful, but I'm pretty sure she sold the books as a trilogy, so the ending of the first book is left wide-open.  The heroine is literally walking down a road, beginning a new adventure as she searches for the truth.  Unfortunately, I can't really bank on the fact that I can sell all three of my books in one go.  From what I understand, as a debut author, it's often easier to sell the first one, see how it does, and then sell the remaining two books.  I need the ending of my first book to be a little more conclusive, a little more definite, with just a hint that there could be more to come, in case it ends up being the only one that gets published (sad, but possible).

There are people that will argue that writing is an art and I therefore shouldn't be concerned about the market or whether the books will be published.  They will say that I should focus on achieving my own vision, not appeasing "the man".  They're right to an extent.  I should be focusing on the story, and I should be working toward the vision in my head.  I need to make the book as good as it can be before I worry about the business end of things.  That's all true.  However, those of you who know me, know that I'm too darn practical for that.  I was a business major after all.

Writing is a business, whether you like it or not, and I want my books to be published.  Yes, I'm working hard to make my novel the best that it can be.  Yes, I am striving to achieve the vision in my head and tell a fantastic story.  But I want people, beyond my immediate friends and family, to have the opportunity to enjoy that story, and the only way to do that is to get it published.  I'm not cutting corners, I'm not pandering to the market, I'm simply trying to consider the business angle while I polish and progress.

Which brings me back to the original point: how am I supposed to end this thing?  I suppose I'll figure it out eventually.  I'm going to have to.  I just have to keep writing, polishing, and perfecting.  It'll get there.  I promise.  Seriously.