Re-read some passages that you’ve already seen and apply my checklist of opinions, structure, and purpose.
Print off a passage and underline keys words such as however, therefore, etc.
Play the game of read-and-cover. Cover up a passage after you read it and try to articulate the main points, including its opinions, structure, and purpose.
Use the “headline” technique to condense the main point of a passage. You could apply this to non-GMAT texts: find a few examples from your favourite website or journal and copy the text into a document, read it, make a headline, and then compare yours to the original.
Read my blog article about the skills involved in Reading Comprehension.
In terms of timing, find a balance between giving yourself enough time to read thoroughly and getting used to reading under time pressure. Set a clock as you read a passage and check how long it takes you.