One of my favorite books on writing is William Zinsser's On Writing Well. In Chapter 10, "Bits and Pieces," he gives a lot of good advice on various aspects of writing. Here are some of my favorite bits and pieces:

On verbs: 

Use active verbs unless there is no comfortable way to get around using a passive verb. The difference between an active verb style and a passive-verb style—in clarity and vigor—is the difference between life and death for a writer. "Joe saw him" is strong. "He was seen by Joe" is weak. The first is short and precise; it leaves no doubt about who did what. The second is necessarily longer and it has an insipid quality: something was done by somebody to someone else. It's also ambiguous. How often was he seen by Joe? Once? Every day? Once a week? A style that consists of passive constructions will sap the reader's energy. Nobody ever quite knows what is being perpetrated by whom and on whom.

On little qualifiers: 

Prune out the small words that qualify how you feel and how you think and what you saw: "a bit," "a little," "sort of," "kind of," "rather," "quite," "very," "too," "pretty much," "in a sense" and dozens more. They dilute your style and your persuasiveness.  

Don't say you were a bit confused and sort of tired and a little depressed and somewhat annoyed. Be confused. Be tired. Be depressed. Be annoyed. Don't hedge your prose with little timidities. Good writing is lean and confident.  

On the exclamation point: 

Don't use it unless you must to achieve a certain effect. It has a gushy aura, the breathless excitement of a debutante commenting on an event that was exciting only to her: "Daddy says I must have had too much champagne!" "But honestly, I could have danced all night!" We have all suffered more than our share of these sentences in which an exclamation point knocks us over the head with how cute or wonderful something was. Instead, construct your sentence so that the order of the words will put the emphasis where you want it.

On transitions: 

Learn to alert the reader as soon as possible to any change in mood from the previous sentence. At least a dozen words will do this job for you: "but," "yet," "however," "nevertheless," "still," "instead," "thus," "therefore," "meanwhile," "now," "later," "today," "subsequently" and several more. I can't overstate how much easier it is for readers to process a sentence if you start with "but" when you're shifting direction. Or, conversely, how much harder it is if they must wait until the end to realize that you have shifted.  

Many of us were taught that no sentence should begin with "but." If that's what you learned, unlearn it—there's no stronger word at the start. It announces total contrast with what has gone before, and the reader is thereby primed for the change.

On concept nouns: 

Nouns that express a concept are commonly used in bad writing instead of verbs that tell what somebody did. Here are three typical dead sentences:  

      The common reaction is incredulous laughter.  

      Bemused cynicism isn't the only response to the old system.

     The current campus hostility is a symptom of the change.

What is so eerie about these sentences is that they have no people in them. They also have no working verbs—only "is" or "isn't." The reader can't visualize anybody performing some activity; all the meaning lies in impersonal nouns that embody a vague concept: "reaction," "cynicism," "response," "hostility." Turn these cold sentences around. Get people doing things:  

     Most people just laugh with disbelief.

     Some people respond to the old system by turning cynical; others say. . .

     It's easy to notice the change—you can see how angry all the students are.  

My revised sentences aren't jumping with vigor, partly because the material I'm trying to knead into shape is shapeless dough. But at least they have real people and real verbs. Don't get caught holding a bag full of abstract nouns. You'll sink to the bottom of the lake and never be seen again.

On "The Quickest Fix": 

Surprisingly often a difficult problem in a sentence can be solved by simply getting rid of it. Unfortunately, this solution is usually the last one that occurs to writers in a jam. First they will put the troublesome phrase through all kinds of exertions— moving it to some other part of the sentence, trying to rephrase it, adding new words to clarify the thought or to oil whatever is stuck. These efforts only make the situation worse, and the writer is left to conclude that there is no solution to the problem—not a comforting thought. When you find yourself at such an impasse, look at the troublesome element and ask, "Do I need it at all?" Probably you don't. It was trying to do an unnecessary job all along—that's why it was giving you so much grief. Remove it and watch the afflicted sentence spring to life and breathe normally. It's the quickest cure and very often the best.

On rewriting:

This is probably his best advice in the entire chapter. I posted some of Zinsser's thoughts from this section here.

Rewriting is the essence of writing well: it’s where the game is won or lost. That idea is hard to accept. We all have an emotional equity in our first draft; we can’t believe that it wasn’t born perfect. But the odds are close to 100 percent that it wasn’t. Most writers don’t initially say what they want to say, or say it as well as they could. The newly hatched sentence almost always has something wrong with it. It’s not clear. It’s not logical. It’s verbose. It’s klunky. It’s pretentious. It’s boring. It’s full of clutter. It’s full of clichés. It lacks rhythm. It can be read in several different ways. It doesn’t lead out of the previous sentence. It doesn’t . . . The point is that clear writing is the result of a lot of tinkering.

Many people assume that professional writers don’t need to rewrite; the words just fall into place. On the contrary, careful writers can’t stop fiddling. I’ve never thought of rewriting as an unfair burden; I’m grateful for every chance to keep improving my work. Writing is like a good watch—it should run smoothly and have no extra parts. Students don’t share my love of rewriting. They think of it as a punishment: extra homework or extra infield practice. Please—if you’re such a student—think of it as a gift. You won’t write well until you understand that writing is an evolving process, not a finished product. Nobody expects you to get it right the first time, or even the second time.

Most rewriting consists of reshaping and tightening and refining the raw material you wrote on your first try. Much of it consists of making sure you’ve given the reader a narrative flow he can follow with no trouble from beginning to end. Keep putting yourself in the reader’s place. Is there something he should have been told early in the sentence that you put near the end? Does he know when he starts sentence B that you’ve made a shift—of subject, tense, tone, emphasis—from sentence A?

Learn to enjoy this tidying process. I don’t like to write . . . but I love to rewrite. I especially like to cut: to press the DELETE key and see an unnecessary word or phrase or sentence vanish into the electricity. I like to replace a humdrum word with one that has more precision or color. I like to strengthen the transition between one sentence and another. I like to rephrase a drab sentence. . . . With every small refinement I feel that I’m coming nearer to where I would like to arrive, and when I finally get there I know it was the rewriting, not the writing, that won the game.

William Zinsser, On Writing Well: The Classic Guide to Writing Nonfiction