Do Not Break Sentences in Two

Try to avoid inserting a period where a comma is needed. For example:

I met him on the train six months ago. Coming from Manchester to London. 

She was an great speaker. A woman who had hunted with Lions and lived with Chimpanzees. 

In both of these examples, the period should be replaced with a comma. 

Sometimes, if dramatic effect is needed, inserting a period can work. For example:

He stood in the darkness screaming her name. No reply.

This needs to be executed well, otherwise it comes across as an error. The place for broken sentences is generally in dialogue, when a character seems to speak in a particularly clipped way. 

The Use of Semicolons

A semicolon should be used when writing two independent clauses as one sentence. For example:

Stephen King's books are horrific; they are so well written. 

It is nearly half past five; we cannot reach town before dark.

These sentences can also be written as two separate sentences:

Stephen King's books are horrific. They are so well written.

It is nearly half past five. We cannot reach town before dark. 

If a conjunction is inserted, then the proper mark is a comma:

Stephen King's books are horrific, for they are so well written. 

It is nearly half past five, and we cannot reach town before dark.

Parenthetic Expressions

A parenthetic expression is an expression that could be placed in parentheses. When using these expressions, a comma should be used.

It is great, when it's windy, to fly a kite up here. 

On a tuesday, when they cook fish, I love the smell.

A name or title when addressing someone directly is parenthetic.

Oh my, Jane, how did you get that?

If you go out looking like that, My Lady, I can't be held responsible. 

Comma Placement

When there are three or more terms in a single conjunction, use a comma after each term except the last. For example:

pears, oranges, and apples

blue, purple, and gold

The only time when this rule isn't followed, is when companies choose to omit it themselves. For example:

Wiseman, Wiseman and Wiseman

Jones, Brown and Deloitte

In this case, see how the company has chosen to write it and follow their lead. 

Adding 's

's should be added to for the possessive singular of nouns, regardless of the final letter of the noun.

e.g.

James's watch

Charles's kitchen

The teacher's notes

There are exceptions. These are when using ancient proper names ending in es or is. 

Jesus' temple

Moses' laws

Isis' palace

Often, these are written as

The temple of Jesus

The laws of Moses

The palace of Isis

The pronominal possessives - hers, its, theirs, yours and ours, don't have an apostrophe. Indefinite pronouns do if they are showing possession.

one's calculator

somebody else's cookie

It's, means it is. Its means, it belongs to.

It's unwise to poke a bear when it's eating its food.

Authonomy - A Great Resource for Writers

Authonomy is a website run by HarperCollins - http://authonomy.com - You can upload part, or all of your novel on here. You have to upload at least ten thousand words to make your work 'go live'. Once it's live, it is available for everyone to see and comment on. 

A new book will start at the bottom of the rankings and will climb depending on how many users back it, comment on it, and rate it. If, by the end of the month, your book has made it into the top five, HarperCollins promise to read the first ten thousand words and offer a critque, and possibly, publication. A free professional review!

To get into the top five takes a lot of hard work. Some users try the approach of messaging everyone to ask for backing. This method, surprisingly, does see them climb up the table, but it seems that to get into the top five, you have to offer to review the work of others to get the same in return.   

However, while a HarperCollins review is nothing to be sniffed at, there is a huge amount of value in joining a reading group on this site. There are plenty of active users on the forums that are more than happy to critque your work for the same in return. As a writer, this is what I want. I would hate to get to the editor's desk before my work's ready, and the community are helping me improve my work every day. To have a place where I can get honest and constructive criticism for free is amazing. All of the people I have met so far have been wonderful, and very kind with their criticism, while offering something useful for me to work with. My book is so much better for it.  

Again, there are people who seem to want to progress up the charts and will praise your work to receive the same in return, but if you find a good group, you will meet a great collection of people that will help you improve as a writer. Here is my novella - http://authonomy.com/books/49125/back-to-basics/ - and here is a link to the group that I am a member of - http://authonomy.com/forums/threads/105135/cool-writers-only-group-cwog-/

I hope to see you on the site.