There are five things you need to write good sentences.  They are:

Imagination– clearly picture what your characters are doing so you can describe it in a way that’s easy to understand

Large Vocabulary– vary your words to avoid repetition and pick the best word to describe an action

Grammar– of course proper grammar and spelling make the words flow together and makes it easy to read; poorly constructed sentences mean the reader has to go back and read it over a few times to get it, thus interrupting them from being immersed in the book

Punctuation– putting a comma in the right place can make all the difference in the world; colons and semi-colons are the boss when it comes to placing the right emphasis and drama on the right words and sentences.  Punctuation is the voice and tone of the words, so choose carefully.

Balance– like vocabulary, vary your sentence structure, and keep a balance between dialogue, description, action, etc.

Imagination and vocabulary go hand-in-hand.  You have to be able to imagine how your character is doing something, then be able to pick the right word to describe it.  There’s a difference between poke, push, and prod, isn’t there?  Each word suggests a different attitude, and thus can tell the reader how the character is feeling or acting.  You pull the right word out of your imagination and the reader will be able to imagine it too.  Also, instead of using ‘said’ all the time, you can use ‘mutter’ or ‘mumble’ or ‘reproached’ or ‘praised’ or any word that fits with how the character’s saying something.

Grammar and punctuation are also buddies.  I mean, obviously they have a lot to do with each other.  Grammar determines punctuation and punctuation determines grammar.  If you don’t know how to use them, they’ll fight a lot, and sentences will end up badly damaged in the fray.  But if you use them right, they will be your most powerful weapon.

Balance is the four of them coming together to make a beautiful sentence.  Varying sentence structure is simply changing who leads the dance between grammar and punctuation.  For instance, there’s two ways of making this statement: 1. She picked flowers and plucked their petals as she walked to school. 2. As she walked to school, she picked flowers and plucked their petals.  If in one paragraph you have all type 1 sentences or all type 2 sentences, the writing is going to come off as stale and slow.  There are other styles of course, and an infinite amount of combinations, so try to use as many different ones as you can.  Meanwhile, imagination and vocabulary are playing together to combine dialogue and description, etc. in an interesting way.  Imagine when your character is talking and walking, and use your vocabulary to make the transition smooth.

One last note: sentences compose paragraphs, and paragraphs also need balance.  Deciding when to end one paragraph and start another, as with punctuation, will determine the voice and tone of the paragraph.  Ending a paragraph with a particular sentence can really bring home the drama.  How you group sentences into a paragraph is up to you; each writer has their own style.  If you’re any kind of a reader, you’ve already been (unconsciously or consciously) studying the technique.

So there you are: the five secrets to writing a good sentence.