To add coherence - what many students call "flow" - to your paper, you need to use transitions appropriately. Transitions can be as brief as a single conjunction that starts a new sentence, or as long as a clause, or even a sentence.
When using transitions between paragraphs, try to base your transitional phrase around the relationships between ideas, rather than using transitions that enumerate your points (first, next, then).
Compare these two topic sentences, one of them revised, from an essay about the roots of the Reformation:
The first sentence makes the transition from the previous paragraphs, which discussed religious controversies of the 1300s and 1400s, to a new paragraph whose idea is clear: Luther's Bible translation caused widespread change in the Christian Church. By linking the new main idea to the preceding ideas, the author reinforces the organizational logic of his essay, and moves it toward a conclusion.
The second topic sentence is a declarative sentence that has no particular logic to it, and fails to move the essay in a particular direction.