When I write a story, I try to have as much background knowledge about EVERYTHING before I start.
This means,
- Knowing your characters (physically, emotionally, and also try to think up a bit of history (just a small life story) for each character ...yes, even the sub-characters and, to some extent, the sub-sub-characters.
This will help you add depth to your characters so they don't just seem like randomly inserted people. It also helps with the plot because you can use their personalities and life stories to "add fuel to your fire". Having a little personal info for each character will help you figure out how the characters interact...what their motives are...why they do what they do. (It also adds a little spice to the more boring parts).
After that, I try to look at each character individually and see how the plot line and the story will affect them personally. (Even if they are a sub-character).
I know it's a lot of work, but it really helps to keep you focused and keep you on track so you don't take the story too far (you need to have some boundaries, or the reader will get lost and lose interest).
When you have everything planned out before you start, it is easy to see where the loose ends are that need to be tied up, so your conclusion will be more complete. People don't like a lot of unanswered questions.
If I try to write a story blindly without having these things planned out, I quickly run out of steam and get bored with it. But when I have little tidbits of info about each character, I know how they can use that info to add to the plot of the story and that makes the story more believable. It also gives the story more dimension.
Then I take all those notes and plot out each chapter by writing a few lines about what will happen in each one. (This way I can see how the story plays out and make sure it will run smoothly. I also do this to make sure there are no inconsistencies in my story.)
With the chapters all plotted out, it is easy to write the rest of the story because I just think of each chapter as it's own short story, rather than look at it as writing a long novel. This makes the "boring" parts more exciting because I know that what I am writing now is important because it is leading to (or concluding) the more climatic parts.
Having all this information on hand will also decrease the likelihood of getting writer's block.
It's hard work, but so worth it in the long run!