The following document from Roane State Community College has been a very helpful one that I often refer to in my writing. It's a good refresher on the basics of the wonderful yet oft neglected punctuation mark, the comma...
The Online Writing Lab (OWL) at Roane State Community College
Connecting Words and How to Punctuate Them
Oftentimes students write in short sentences without connecting the sentences together. This creates a disjointed and choppy effect, and frequently gets in the way of showing connections between ideas, as well as sentences. Occasionally students who write this way also fall into the habit of creating inadvertent sentence fragments.
To create a better flow, four types of connecting words can be used: conjunctive adverbs, subordinating conjunctions, coordinating conjunctions, and transitions. The words which fall into these categories, however, may cause confusion for students because they don't know how to punctuate them properly.
By reviewing the lists of words below, keeping the lists handy, and following the guidelines for punctuation, students can improve their writing dramatically.
1. Conjunctive Adverbs
The first type of connecting words are conjunctive adverbs (to "conjunct" simply means to "join"). Conjunctive adverbs often fall in mid-sentence. If you join two "independent clauses" (i.e., sentences) using one of these words, use a semicolon before it and a comma after it. (You end a sentence with a period, right? So if you join two sentences together with a comma, you're splicing them together. That's called a comma splice and is a serious error. Two sentences held together with duct tape (i. e., the comma), simply won't hold).
Correct example: I prefer to eat my dinner at the dining room table; instead, I usually eat in front of the television. (The word "instead" is a conjunctive adverb.) Incorrect: I prefer to eat my dinner at the dining room table, instead, I usually eat in front of the television. (This sentence contains a comma splice. The writer tries to join two separate sentences with a comma before "instead.")
When you see a conjunctive adverb in the middle of a sentence, a red flag should go up! Here's a list of conjunctive adverbs:
also anyway consequently finally furthermore hence however incidentally indeed instead likewise meanwhile moreover nevertheless next nonetheless otherwise still then therefore thus
2. Subordinating Conjunctions
Second, are subordinating conjunctions. (To subordinate implies that something is of less importance than something else or that something depends on something else.) If a subordinating conjunction falls in mid-sentence, there is usually no comma used. If it starts a sentence, there is a comma after the introductory phrase (right before the subject and verb).
Correct example: After eating dinner in front of the television all week, I decided to change locations. (The word "after" is a subordinating conjunction. Here it begins a sentence, leading to an introductory phrase. Notice that the comma falls right before the subject and verb.)
Compare: I decided to change locations after eating dinner in front of the television all week. (Notice that there is no comma before the "after" in the second example. Because it falls in mid-sentence, there is no introductory phrase which would require a comma.)
Here's a list of subordinating conjunctions:
after although as as [far/soon] as as if as though because before even if even though how if inasmuch as in case [that] in order that insofar as in that lest no matter how now that once provided [that] since so that supposing [that] than that though till unless until when, whenever where, wherever whether while why
3. Coordinating Conjunctions
Third, are coordinating conjunctions (of equal importance instead of lesser importance). Put a comma before a coordinating conjunction only if it joins two independent clauses (i.e., if there is a subject and a verb on either side of the conjunction). If a subject or a verb is lacking on either side of the conjunction, there is no comma.
Correct example: I ate dinner in front of the television every night, and I finally got tired of it. Compare: I ate dinner in front of the television every night and finally got tired of it. (This sentence lacks a subject after the "and," so there is no comma.
List of coordinating conjunctions:
and so yet but or for nor
***If your high school English teacher told you that coordinating conjunctions always had commas before them, he/she was wrong. Get over it! And if the same teacher told you that you couldn't start a sentence with "and" or "but," then he/she was also wrong. Look it up in your handbook. "And" and "but" are very effective transitions when used properly and not overused.
4. Transitional Phrases
Finally, there are transitional phrases, which usually begin a sentence and are followed by a comma:
after all as a result at any rate at the same time by the way even so for example in addition in fact in other words in the second place on the contrary on the other hand
Use transitional phrases between paragraphs to create a logical flow in your writing. Use them within paragraphs, too. Avoid overusing a particular phrase. Learning how to use these words will help you communicate clearly and with emphasis. But take care to punctuate them correctly because incorrect punctuation will frequently confuse your reader or even change the meaning of a sentence. And avoid archaic transitional phrases, such as "hitherto," "whereof," "whereby," and "whereas."