As an instructor and as someone who writes my thoughts down regularly, I try to continually educate myself on proper grammar, sentence structure, and other issues in the English language (I will deal with my use of the comma before "and" in another post). I have noticed THAT I have spent a great deal of time talking to students about the use of that word "that."
It has been my experience that most of the time, when "that" is used, it is really not needed, it can be simply eliminated from the sentence, and the sentence would read just as well. However, there are times it must be used. So I set out on a quest to find when and when not to use the word "that." I came across some helpful articles.
This article shows THAT the sentence,
"He said that that 'that' that that man used was wrong" is a grammatically correct sentence and why it is so.
This article is a brief summary of the rules of when to use "which" and when to use "that."
This article gives the rules again but in a more technical sense (restrictive and non-restrictive clauses).
Sometimes when I read students papers, I can mark out about 80% of their uses of the word "that" and it reads great. However, these articles have helped me judge when it is time to use THAT word and when it is not.
I hope THAT this has helped you, too.