I said a couple of times during my recent trip to UPenn that "Most writing is too long". People seemed to nod appreciatively at this, so here's some more on that topic...
Most writing is too long and the most common reason is that it's not written for the reader's benefit. Readers want the important stuff, as clearly as possible, in the shortest possible space. If you remember that and let it guide your writing, you won't go far wrong. The reader's favourite bits are the ones you don't write.
The problem is that it's tempting to write for your own benefit, not the reader's, and this almost always ends up making things too long. This can take many forms:
Some write to help themselves understand the material, such that the end product is a record of their learning process. Others will insert details that the reader doesn't need, because it's a topic the writer's fond of. Other fear making tough decisions about what to include, so they say everything and hope some of it's good: "Write it all and let God sort them out."
This is common because formal education teaches you to write poorly. Specifically, it encourages people to overwrite. Teachers and professors give assignments and they set a minimum word count. This sends the message that where writing's concerned, more is better.
Teachers have their reasons. They want a brain-dump to show that the student's done the homework, pretty much the opposite of good writing. That's fair enough for school, but if you internalize that philosophy, you will end up writing to show off rather than for the reader's benefit.
Once you put the reader's interests first, you'll naturally start to find your own ways to achieve that. Everyone's style is different, but here's a few I've learned:
If it starts "On that note...", "Also...", or "Furthermore...", you should probably cut it.
Join Twitter - writing to a 140 character limit is a great form of discipline. Then imagine that every paragraph you write must become a tweet. You may find you can compress that paragraph into one sentence.
Just as Twitter is good, other artificial constraints are good. Set yourself a word limit; if you're blogging, make it 500 words.
Unless the article's about you, sentences that include the word "I" or "we" can usually be cut.
Think of your piece as a nuclear missile: it has a payload, the message you want the reader to grasp, and a rocket motor, the introduction and other stuff you need to ensure it reaches the reader. Every missile needs a motor, but designers try to make the payload as big as possible, given the size of the motor. Identify what your payload is, and what your motor is. Then think, is my motor too big? (It probably is.) In this paragraph the missile analogy is the motor.
As a rule of thumb, by writing it better, you can cut it down by half.