What is a paragraph?
A paragraph is a collection of related sentences dealing with a single topic. Learning to write good paragraphs will help you as a writer stay on track during your drafting and revision stages. Good paragraphing also greatly assists your readers in following a piece of writing. You can have fantastic ideas, but if those ideas aren't presented in an organized fashion, you will lose your readers (and fail to achieve your goals in writing).
The Basic Rule: Keep one idea to one paragraph
The basic rule of thumb with paragraphing is to keep one idea to one paragraph. If you begin to transition into a new idea, it belongs in a new paragraph. There are some simple ways to tell if you are on the same topic or a new one. You can have one idea and several bits of supporting evidence within a single paragraph. You can also have several points in a single paragraph as long as they relate to the overall topic of the paragraph. If the single points start to get long, then perhaps elaborating on each of them and placing them in their own paragraphs is the route to go.
Elements of a paragraph
To be as effective as possible, a paragraph should contain each of the following: Unity, Coherence, A Topic Sentence, and Adequate Development. As you will see, all of these traits overlap. Using and adapting them to your individual purposes will help you construct effective paragraphs.
Unity
The entire paragraph should concern itself with a single focus. If it begins with one focus or major point of discussion, it should not end with another or wander within different ideas.

Writing transitions
Good transitions can connect paragraphs and turn disconnected writing into a unified whole. Instead of treating paragraphs as separate ideas, transitions can help readers understand how paragraphs work together, reference one another, and build to a larger point. The key to producing good transitions is highlighting connections between corresponding paragraphs. By referencing in one paragraph the relevant material from previous paragraphs, writers can develop important points for their readers.
It is a good idea to continue one paragraph where another leaves off. (Instances where this is especially challenging may suggest that the paragraphs don't belong together at all.) Picking up key phrases from the previous paragraph and highlighting them in the next can create an obvious progression for readers. Many times, it only takes a few words to draw these connections. Instead of writing transitions that could connect any paragraph to any other paragraph, write a transition that could only connect one specific paragraph to another specific paragraph.

Transitional Devices
Transitional devices are like bridges between parts of your paper. They are cues that help the reader to interpret ideas a paper develops. Transitional devices are words or phrases that help carry a thought from one sentence to another, from one idea to another, or from one paragraph to another. And finally, transitional devices link sentences and paragraphs together smoothly so that there are no abrupt jumps or breaks between ideas.
There are several types of transitional devices, and each category leads readers to make certain connections or assumptions. Some lead readers forward and imply the building of an idea or thought, while others make readers compare ideas or draw conclusions from the preceding thoughts.
Here is a list of some common transitional devices that can be used to cue readers in a given way.
To Add: and, again, and then, besides, equally important, finally, further, furthermore, nor, too, next, lastly, what's more, moreover, in addition, first (second, etc.)
To Compare: whereas, but, yet, on the other hand, however, nevertheless, on the contrary, by comparison, where, compared to, up against, balanced against, vis a vis, but, although, conversely, meanwhile, after all, in contrast, although this may be true
To Prove: because, for, since, for the same reason, obviously, evidently, furthermore, moreover, besides, indeed, in fact, in addition, in any case, that is
To Show Exception: yet, still, however, nevertheless, in spite of, despite, of course, once in a while, sometimes
To Show Time: immediately, thereafter, soon, after a few hours, finally, then, later, previously, formerly, first (second, etc.), next, and then
To Repeat: in brief, as I have said, as I have noted, as has been noted
To Emphasize: definitely, extremely, obviously, in fact, indeed, in any case, absolutely, positively, naturally, surprisingly, always, forever, perennially, eternally, never, emphatically, unquestionably, without a doubt, certainly, undeniably, without reservation
To Show Sequence: first, second, third, and so forth. A, B, C, and so forth. next, then, following this, at this time, now, at this point, after, afterward, subsequently, finally, consequently, previously, before this, simultaneously, concurrently, thus, therefore, hence, next, and then, soon
To Give an Example: for example, for instance, in this case, in another case, on this occasion, in this situation, take the case of, to demonstrate, to illustrate, as an illustration, to illustrate
To Summarize or Conclude: in brief, on the whole, summing up, to conclude, in conclusion, as I have shown, as I have said, hence, therefore, accordingly, thus, as a result, consequently