(I apologize for the typos in the following examples. I made them up myself, and I'm no native speaker!)
Two main techniques to overcome paragraph sprawls:
1. Commitment and response. You tell the reader you are going to talk about something, then stick to your promise.
Example: Many people have been speculating over the causes that led to Leslie Cheung's tragic suicide. Some say it was a lovers' quarrel; others think the superstar had been suffering from an ongoing depression; and there are still those who attribute the whole affair to some unaccountable jinx that befalls a celebrity every 10 years.
(I am making it up! I really don't know why he died)
Then offer your own interpretation.
In this example, I promise, in the first sentence, to talk about the possible causes of Leslie's death, followed by the gossip itself.
Example . Until the awe-inspiring performance at the 1998 Olympic games, Chen Lu had given every indication that she was "history" in the world of figure skating. Just the previous year, she did not even qualify for the long program at the World Championship. She had gained weight; she had lost her coach; her spins travelled for a mile; and her triple jumps -- for a long time a staple in her programs --had deserted her. Coming into the 1998 Olympic games, no one in the skating world was giving her much of a chance, except, perhaps, Chen Lu herself.
In this example, I tell the reader that no one expected Chen Lu to medal in the 1998 Olympics, and then I explain why.
Example
Inanimate objects are classifed into three major categories--those that don't work, those that break down and those that get lost. (Quote from Russell Baker, "The Plot Against People")
In this paragraph, Russell tells his readers he is talking about three objects, and he explains what they are. Then he is going to discuss each thing one by one. (Note the rule of three in the examples!)
You can also start with a question, then answer it yourself.
Example
Why did Leslie commit suicide? Everyone in HK was wondering about this. And no one really knows for sure. Some say…some say…
Then offer your interpretation.
2. Second Technique: Downshifting -- going from talking generally to talking specifically
You start the paragraph with a general statement, then you provide explanation, statistics and examples (i.e. the details).
If you give a score of 10 to the most general statements, and 1 to the most specific ones, then your paragraph should start from 10 and work towards 1.
Example
10. For the past decade, the educators in the U.S. have seen a steady decline in the quality of Math education a student receives in an American public school.
9 The latest data release from the TIMSS study has, once again, reminded the American public of just how "fuzzy" their Math is (to quote George W.). (proof)
8 According to the report, U.S. students, by 12th grade, are way behind students in other countries in all areas of Mathematics. (be specific)
7 According to Dr. Whatabore, a professor at University of ABC, the Math curriculum in many schools is "a mile wide, and an inch deep" -- as a result, students graduate from high schools not knowing much of algebra, geometry or trigonometry.
In this example, I make a statement, then I use statistics to back up my statement, and then I close the paragraph with a quote from a professor. The paragraph goes from the most general (with a score of 10), to the most specifc (a score of 7).