The most basic block of any piece of writing is the paragraph.  Paragraphs are specific to writing; we speak in sentences, but we don't generally speak in paragraphs.  There's nothing spontaneous about them--paragraphs reveal the premeditated nature of writing. They're constructed; writers organize their material into the units that make sense for any particular writing project. 

Paragraphs are the structural workhorses of writing; I like to think of paragraphs as the employees of a paper, and as such, each needs to have a job description.  The more conventional term is "topic sentence"--the sentence that makes clear the purpose of the paragraph, its main point. Not all paragraphs have explicit topic sentences, of course, although I'd recommend you write a topic sentence for each paragraph if you're still an anxious and somewhat inexperienced writer. Aside from helping you identify what you want the paragraph to accomplish, they help the reader by providing an overview or the paragraph's intentions.  With or without an explicit topic sentence, however, you should be able to clearly state what each paragraph does for your paper--how it contributes to your argument, how it relates to the thesis driving the entire paper, how it moves the paper, and therefore the reader, closer to understanding your overall point. In other words, you should be able to state what makes any given paragraph a necessary employee who contributes positively to the workplace.  As with employees, no paragraph should be expected to do more than a single paragraph can efficiently accomplish; likewise, no paragraph should be allowed to just hang out by the water cooler, taking up space and trying to look busy, or to just repeat the efforts made by another paragraph. 

Once you've drafted a paper, you can craft a paragraph-by-paragraph outline.  Write a one-sentence summary of each paragraph, leaving a space between each sentence. Don't use your existing topic sentences--rather, reread the paragraph and write a sentence that genuinely summarizes its point (you might want to compare it to your existing topic sentence, however, to see how well your intended topic sentence actually covers the paragraph you wrote).  Focus on the job the paragraph accomplishes--what's the critical idea here the reader needs to take away?

You can then compare the summary sentence to the paragraph as a whole.  Are there random sentences that don't really relate to the summary sentence? Did you drift as you got to the end of the paragraph? Did you add a sentence at the end that anticipates your next paragraph--a strategy some high school teachers recommend, but which results, finally, in papers that sound a wee bit crazy? (Those sentences read like non-sequiturs. They rarely work.)

When you've written a summary sentence for each paragraph, you can assess your draft as a whole. Read through all your summary sentences and try to follow the logic that gets you from one paragraph to the next.  Try inserting transitional words between the summary sentences that indicate the logical relationship that connects one paragraph to the next.  Some examples of transitional words: "furthermore," "in addition," "indeed," "and" (all of which express a continuation or an intensification of a particular idea); "however,"  "but," "yet" (indicating a change in the reasoning, consideration of an opposing view or contradictory evidence); "nevertheless" "just the same" (returning to the main line of argumentation); "consequently," "therefore," "so" (introducing the results or conclusions to be drawn from the line of reasoning being pursued); "because," "since" (indicating a cause-and-effect relationship between two ideas); "for instance," "for example" (introducing specific illustrations of the previously stated idea).  

Constructing a paragraph-by-paragraph summary outline allows you to play with structure efficiently, without immediately carving up your actual paper.  And if you create your outline on the computer, it's easy to shift the sentences around in the list and change the transitional words connecting them to try on different structures. 

A paragraph-by-paragraph outline will also help you to diagnose the coherence of your individual paragraphs: Can this paragraph really be summarized in a single sentence? Does that sentence really cover all the ideas conveyed in the paragraph? And by extension the outline will also help you assess the coherence of your paper as a whole: Does it make sense to tackle these ideas in the order that I've chosen?  Is the logical relationship between my paragraphs clear? Might some other order make more sense?

Finally, you should be able to relate each of your summary sentences back to your thesis. The sentences must bear some relationship to the overall point you're making. If you include a paragraph about rhyme scheme in a paper about a poem, but you fail to demonstrate how that rhyme scheme relates to your claim about the poem's meaning, then that paragraph has no reason for being--delete it.

Thinking in "Blocks" 

You can use the idea of a block--a unit of meaning composed of one or more paragraphs--to look at larger units of meaning and purpose in your writing.  Blocks are determined not by length, but by intention; related paragraphs (for instance, in your paragraph-by-paragraph outline, a group of summary sentences all joined by transitional words that indicate continuation or intensification of an idea) constitute a single block.  Like paragraphs themselves, such blocks don't exist in nature--they're not out there for you to discover.  You create them, and you do that by organizing your material around specific purposes that you consciously construct. 

If you can't find blocks in your paper--if every single paragraph seems to shoot off in some new direction, entirely unrelated to the preceding paragraph--you're probably stringing together a series of random observations that aren't building efficiently towards a thesis or a main point.  Go back through your paper and look for paragraphs that deal with related ideas; you might also have to break down existing paragraphs if it's clear to you that they're not really paragraphs (those random acts of indentation).  You might want to use different colored highlighters to identify sentences that can be recombined into more effective paragraphs, and paragraphs that can be recombined into effective blocks.  You can then use the paragraph-by-paragraph outline technique to play around with the overall structure of your paper.

Ultimately, an inability to clump your ideas into blocks might mean the paper lacks purpose--that is, your thesis is still under-developed.

Don't be timid about playing around with blocks, once you've begun to recognize them or construct them.  Would beginning your paper unconventionally--that is, with a block the reader might not expect in the beginning--provide an effective hook for a reader?  Is the block you've constructed as your conclusion actually better as an introductory block?  Do you want to open your paper with a block that covers the opposing view--and then devote the rest of the paper to refuting that view?  Or do you want to make a passing nod to opposing viewpoints only at the end of your draft?  How would each approach affect the reader of your draft?  There's no single way to "correctly" structure a piece of writing; different structures will evoke different responses from the reader.