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.