The only thing harder to write than introductions and conclusions are transitions between paragraphs and sentences. But they’re essential to good writing because they help guide your readers smoothly through the logical development of your ideas.
Transitions between paragraphs act like little conceptual “bridges.” You don’t want your paragraphs to be like a strand of separate islands, each of which stands conceptually isolated from the islands on either side of it. You need bridges to get from one to the other, and so on. This is what transitions between paragraphs do.
Transitions within paragraphs string the ideas within sentences together. One way of helping to make these sentences flow better is to first get them in the right order. Think of each paragraph as a little story. Each story has a natural beginning, middle, and an ending, either chronologically or logically speaking. Getting your sentences in the right order will greatly aid you in writing transitions within your paragraphs.
For a more detailed discussion of transition strategies and devices I strongly recommend you check out this link.