First You Write The Book!

How can a writer ensure a story has a good beginning, a satisfying ending and good continuity in between?  

This is a tough question! But  I have a few thoughts. 

You have to get the book, no matter how awful, on paper.  It's mandatory to have something to work with.  I have a friend who writes beautifully, and she revises what she writes until she has a finished product when the book is done.  That's not how most of us do it.  Most of us product something called, excuse me, "the shitty first draft."  
So, you have the story from beginning to end, but it doesn't quite hang together, and it obviously needs work.  Several options here.  My favorite is 
a) The Writing or Critique Group.  Other writers can help.  They can read your story and offer their opinions.  Some of these you won't like.  But if you think hard about them, you will see the validity.  And you are free to take all of the advice, some of the advice, or none of the advice. It will be more helpful is some of these writers are writing in your genre. 
b)  Writing is re-writing.  Take a course or two on self-editing.  
c)  Read books on editing, And on writing on general.  Try to follow the rules.  Most of the time. Remember, rules are made to be broken.  Infrequently.  When you understand what you are doing.  For heaven's sake, don't be slavish to rules.  Currrently, "no adverbs" are a rule.  Break if occasionally.  Get rid of "just," and "suddenly," and "some" and all the weasel words in your first draft.  
d). Remember the classical plot has a beginning, middle and an end, and understand how much space each needs in your manuscript.  Read Robert McKee's STORY.  Learn a little about screen plays.  Most novels follow the basic rules. 
e)  Read amd re-read what you have written.  Prune words, sentences, sometimes whole scenes.  Make it tight.  Don't state the obvious, don't repeat yourself. 
 f). Increase your vocabulary, and write in the vocabulary (mostly) of. your time period. Just be careful
g). Understand that you will make mistakes.  Most can be fixed.  Tighten your plot. Prune the "saggy middle."   Study best-selling novels in your genre.
h). If all else fails, remember you can liven the most pedestrian manuscript, with these few words, "A Man With A Gun Came Throught the Door."  This can be literally (if you are writing genre,) or metaphorically.  Something unexpected happens.  You can always go back and put in hints that it will.  
i). Write bravely.  
j). Join professional organizations in. your genre! 

Good luck

Here are some writers who will have more advice for you.

Skye Taylor http://www.skye-writer.com/blogging_by_the_sea
Marci Baun  http://www.marcibaun.com/blog/
Margaret Fieland http://margaretfieland.wordpress.com
A.J. Maguire  http://ajmaguire.wordpress.com/
Beverley Bateman http://beverleybateman.blogspot.ca/
Rhobin L Courtright http://www.rhobinleecourtright.com