I personally find it rather hard to write endings. And beginnings. And middles. Plot... plot in general is not my strong suit. Yeah, I know that's not exactly a stellar point in my favor. It's something I'm trying to work on.
That being said... I feel the biggest problem when it comes to writing a good ending is that a lot of writer's don't entirely realize the expectation they set in the reader's mind. The ending's a sort of "goal point" for the reader, it's something that's always in their mind, something they always know is coming. And they know, more-or-less, what will herald it - the central conflict being resolved. So, with that in mind, they begin to develop their own idea of the ending, and what it should be. An idea that often conflicts with what the writer has in mind - this problem is considerably worse for long-running series. Even still, it's not impossible to write a good ending even with reader expectations, I think all it really needs is for the writer to remember that they shouldn't pull the ending out of their ass. Be conscious of the story you're writing, of what you're either explicitly telling the reader or implying through use of tropes and archetypes. Don't try to be clever, or atleast, don't go out of your way to be clever. An ending should be a natural conclusion to all the themes, concepts, and obstacles set out over the course of the story. It should bring closure to all the character arcs established throughout the series. What it should not be, is a joke pulled on the reader. In my experience, trying to pull the rug out from under them rarely works. It just leaves the reader feeling cheated.
Lemme give you an example - I write a story about, say, a farmboy who finds out he's the bastard son of a king and sets out to claim his throne from some evil usurper. Not exactly the pinnacle of originality, but bear with me here. Now this farmboy, he has no desire to be king. None at all. But he's the kingdom's only option. So while the physical conflict of the story would be between this farmboy and this usurper, there's a internal and thematic conflict of accepting responsibility and duty, something this farmboy learns over the course of his story. He doesn't learn it all in one arc, or from one character, but over the course of several adventures, as the result of several characters effecting him - this is what ties all the characters and arcs in the story together, how they effect this farmboy's view of the burden placed upon him.
Now, here comes the finale. The farmboy has accepted his duty, he's ready to be king - he's reluctant, but, well, he has to do what's right. And once he's slain the usurper, once he stands in the throne room of his father, ready to claim the throne - a side character comes and says "Hey, I'm the bastard son of the king, too! Turns out he really got around!" And the farmboy goes "Oh, okay, you take the throne then. I'll go be a baker or something."
The reader's gonna feel cheated. Sure, you may not have ever explicitly said that this boy is absolutely going to be king, or that he's the only bastard running around. Maybe you even did a decent job of hinting that there is another heir. But the reader is going to feel cheated. They've spent the entire course of the story getting invested int he struggle of this farmboy, in the conflict he has with himself, and you bring a twist out of the blue for a bit of cheap shock. Sure, some people will say they liked it. Some people will crawl out of the woodwork and crow about realism, how coincidences happen sometimes, how the themes in life don't get neatly wrapped up so why should a story be any different? But it doesn't change the fact that you completely betrayed your story. Because that twist? It may be clever. It may be "realistic." It may work on paper. But there's a whole lot more to stories than that. There's themes, character growth, consistency. This is something may writers today, I feel, simply do not understand. They're so concerned with plot, with the physical happenings, that they often forget thematic importance - which goes a long way to leaving your reader with a satisfactory ending.
So, that's what I recommend. Consider how well you handle themes and growth, and then see how you can tie that up with your ending. And your beginning, for that matter - they both should reflect the entirety of the story.