"Coherence," or "flow," refers to the smooth connection between clauses, sentences, and paragraphs. This includes the idea that all sentences and paragraphs are arranged in the correct order, but usually means that different parts of your writing fit together logically and meaningfully.
Making the Right Connections
A key part of coherence is the proper use of conjunctions. Conjunctions show the relationship between two clauses, sentences, paragraphs, or groups of paragraphs.
Connecting Parts of Sentences
For example, the conjunction in "He was angry because his car broke down" shows a cause-and-effect relationship—the second clause was the cause of the first clause.
If you write, "I like to finish my homework as soon as possible. My brother, on the other hand, waits until the last minute," the conjunctive adverb "on the other hand" points out a contrast between you and your brother.
Entire paragraphs, similarly, can be connected with conjunctions—I just did so with the word "similarly," which describes a relationship (sameness) between this paragraph and the one or ones before it.
You must not only use the correct conjunctions, you mst also know when not to use conjunctions. Some students use conjunctions too much—especially conjunctive adverbs such as "however" and "therefore"—often at the beginning of every sentence in a paragraph.
Conjunctions should be used only when they are needed for clarity. If you join two clauses or sentences with a conjunction, you should ask yourself: is the relationship between them just as clear without the conjunction? If the answer is "yes," then don't use the conjunction.
For example, the sentence "I like classical music; however, my sister enjoys rock and roll," is just as clear when a semicolon is used: "I like classical music; my sister enjoys rock and roll."
You may still use a conjunction for emphasis, if you think that the relationship is important. Just do not use conjunctions too much!
Sentence and Paragraph Order
Another problem with coherence is in the ordering or sentences or paragraphs.
If you write a paragraph without an outlined plan, it is possible that your sentences will not be ordered very well. Sometimes a student will write about one idea, then change to a different idea, then come back to the previous idea. This is poor coherence.
Similarly, paragraphs may be in a poorly-chosen order. You can have many different ways of ordering sentences, and most will work well. You can order paragraphs by importance, time order, location, etc. You may choose the most important one first, or maybe leave the most important one for last. You may use reverse time order in order to make an event appear more clear (showing effect before cause). All of these are possible, though it is best if you write the sentences and words based upon the order you have chosen. Your style of writing will change based upon the order.
Many students will write body paragraphs in the order they thought them up. This is one of the few bad ways of ordering! So long as you have some good reason for ordering, almost anything is possible.