Starting your story can be the hardest thing in the whole process. The first step in a journey is often the hardest to take and, in terms of your story, if the beginning is not interesting then you have already lost the reader. It's a lot of pressure and everything in your story much be deliberate,, or else you will lack cohesion.
Yet, another problem that comes up is often bringing the story to a close. Once the journey has begun I have found it is much easier to keep trucking along, at least when compared to actually starting. Your characters have goals and motivations. Events have been set in place and, in many ways, the story isn't in your hands. It is as though it is being channeled in this phase.
But there comes a time when a heroes tale must end, and whether you are choosing to kill your hero or marry them off, it has to be satisfying. It has to make sense.
We have all read at least one story that we beautiful all the way up until the end. Maybe the author just didn't give us what we wanted (as is the case for Mass Effect 3) or perhaps the ending felt as though it were wish fulfillment (such as the ending to Naruto) or, hell, maybe it was just a bad ending and the whole damn thing needed fixing.
So, as story tellers, how do we avoid this?
On way to avoid an unsatisfying ending is to have a clear idea of what it is you are trying to say with your story. This may sound like the kind of advice you would get from a high school English teacher, yet it is nonetheless important advice. By knowing what the story is as we write it we are able to have better control of tone, our characters, and plotting. That is not to say that your story should not be allowed to "take you places." It is important to allow the story to come freely and naturally, as difficult as that may be at times. Deliberate does not mean predictable, it simply means that things happened for a reason. At the time of writing a certain passage you may not have thought about how important that would be later on in the story and it is then we must edit as needed.
Knowing exactly what your story is and how you want it to end will help to get away from editing or feeling as though the story started out one way and then ended in a completely different way.
Now, let's say you know what you want to do with your story. You know what you want to tackle with it and you know exactly how you want it to end. Let's say that you write your story and, hold on, there isn't enough room in this book for it to be finished without there being over a thousand pages. Okay, that's fine, oh, but what's that, you are actually going to need another fifteen books to finish one characters story? Oh boy, that may be an issue.
Now, before we get into why having a series that stretches onward into infinity is a bit of a problem, I feel I should clarify. There are exceptions to this rule. I'm a fan of Game of Thrones and Martain does a good job of having an extended series of epic fantasy without it feeling as though the story is endless; The reason for Martain's success in that regard goes back to that planning thing I mentioned about.
There are also authors that do novels set in the same world but use different characters (such as Max Gladstone's Craft Sequence). Those novels are exempt.
So, let's say you need to have your series be the length of The Wheel of Time series in order to really tie everything together. If that is the case then I suggest you re-evaluate your story. I say this because it is important that only the important events are in your story. You may think that this does not effect the ending, however, most readers look for things to be concluded in some way. If you were to write a story and leave hanging plot threads by the conclusion then, ultimately you have failed to deliver a satisfying end to you tale.
Many of us have heard that the journey is more important than the ending and, in terms of writing, that is often the case. What your characters experience and how the world around them influences them and they, in turn, influence it is often the most rewarding part of reading a story. However, once we get to the end it is important to feel as though the journey mattered. If our story is overly long then the conclusion much be all the more epic so as to satisfy the epic tale.
However, if when we come to the end of our story we feel as if the ending makes no sense, or that the author has given us, the readers, the ending we were hoping for despite evidence otherwise, then we feel as if the journey was of no importance. As I've stated many times, your story and the events that unfold within it much be deliberate. If you have no control over what is happening and let the story get too far away from you then you end up writing little more than a random list of events that have little to no impact on each other.
Even in post-modern writings the story ties itself together in a strange way. It does not matter on the path we take when we go from A to B, only that we reach our destination and all the events and plot threads are neatly tied up.