I wrote a blog post yesterday about LSAT diagramming and how to better determine its usefulness for you, where essentially I suggested that diagramming is an investment of your time and rhythm and for that investment to be worthwhile it needed to pay dividends in terms of either (1) gaining you more time than it took as you continue on (like a prephrase, where you can more aggressively/efficiently sort through the answers), or (2) leading to an improved understanding of what you're facing such that you've increased the likelihood of getting the question correct. Failing both of those, as I stated, likely means that your diagramming was probably a poor investment and you would have been better off without it.
However I ended that discussion with the statement, "I'm NOT encouraging you to never diagram!...Rather, I'm encouraging you to consider how much time you've spent making notations, and whether that time was well spent, or could have been better spent elsewhere." With that in mind I thought I'd take a brief moment to give a few tips on how to make any notations that you do choose to show less time consuming and still powerful enough to convey the intended point.
For Reading Comprehension:
A lot of people feel the need to write brief summaries for each paragraph or at the end of the passage. STOP. Feel free to pause occasionally as you read to collect your thoughts, but odds are you won't need to write them down.
It's usually better, both in terms of clarity and speed, to notate outside of the passage text than within it. For instance, brackets beside text are both faster to create and less prone to obscure the words than underlining or circling in the passage itself.
Once you finish reading a passage and making any notations, and after you have answered the questions and analyzed your performance, reconsider what you chose to diagram and think about its utility. That is, how long did it take (approximately), and to what extent did it improve your understanding of the passage and your ability to answer the questions. The point of these considerations is to determine what notations you could have done without, and how much time that would have likely saved you; on future passages then you'll have a better sense of what/when to diagram to maximize its value.
For Logical Reasoning:
As with Reading Comprehension, most LR diagramming is likely to prove unnecessary. Ask yourself questions like, "did I really need to underline or mark the last sentence as the conclusion, or was simply recognizing that the last sentence was the conclusion sufficient?" The key is notating only what you need in order to obtain the greatest degree of speed and insight.
Certain stimuli do tend to be well-suited for diagrams, however: questions with Formal Logic, questions with conditional reasoning chains (A --> B, B --> C, etc.), and even occasionally questions with causality where you can quickly sketch the cause and effect...diagramming in these instances often is worthwhile for a lot of test takers.
For Logic Games:
A main diagram, as well as individual diagrams for certain questions, is required for every Game. The only "exception" to this idea isn't really an exception as much as an adjusted application of it: some Games won't provide much information to help you create a main setup--certain Pattern Games, for instance--and you'll simply sketch shorthand versions of the rules. Even here though you'll still want to notate the variable sets and restrictions given.
That said, you should still practice to represent variable sets, your base/structure, rules, and inferences as efficiently as possible. Go back after completing a Game and consider your setup: which elements within it could have been represented more quickly or effectively and still been clearly understood?
Hopefully you can see that diagramming on the LSAT isn't something to steer clear of, but rather it's a process that should be refined through continued trial and error, and lots and lots of practice. Your goal is always to find that perfect nexus where ultimate efficiency meets complete accuracy; diagramming will certainly play a role in that synthesis, and it is your job to determine exactly how to best incorporate it for your needs.