Transition words and phrases improve coherence by indicating the logical relationship between one word, phrase, sentence or paragraph and one that follows.  They thus enable the reader to follow your argument (or narrative) more easily and accurately.  Below is a list of some transitions, grouped according to their function.  To decide which transition is appropriate, you must ask yourself certain questions:  Is the next thing I want to say an addition to what I've just said?  Or does it set up a comparison or a contrast?  Does it explain a cause or effect?  Does it illustrate a point by using an example?  Is it a clarification?  Am I making a concession?  In short, choose your transitions carefully, according to the logical relationship of the ideas you are connecting with that particular transition.

 Compare the following two paragraphs for flow and ease of comprehension:

       Working in the neighborhood grocery store as a checker was one of the worst summer jobs I've ever had.  I had to wear an ugly, scratchy uniform.  It was cut at least two inches too short.  My schedule of working hours was inconvenient.  My hours changed each week.  It was impossible to make plans in advance.  Getting a day off was out of the question.  The lack of working space bothered me.  Except for a half-hour lunch break, I was restricted to three square feet of room behind the counter.  I felt like a cog in the cash register.

       Working in the neighborhood grocery store as a checker was one of the worst summer jobs I've ever had.  To begin with, I had to wear an ugly, scratchy uniform that was, moreover, cut at least three inches too short.  My schedule of working hours was another inconvenience.  Because my hours were changed each week, it was impossible to make plans in advance, and getting a day off was out of the question.  In addition, the lack of working space bothered me.  Except for a half-hour lunch break, I was restricted to three square feet of room behind the counter; as a result, I felt as if I were no more than a cog in the cash register.

To indicate:

       Addition        furthermore        and
                             also                   again
                             in addition          next
                             besides              finally
                             moreover           what's more

       Examples      for example     specifically
                             for instance      namely

       Comparison     similarly               likewise
                                in comparison      by the same token
                                in similar fashion

        Contrast        nevertheless           though
                              in contrast              while
                              nonetheless            otherwise
                              on the other hand    however
                              yet                         instead
                              although                 rather

       Clarification  in other words
                             that is to say
                             in short (when clarification is also a summary)

       Cause            because         since

       Effect/Result    therefore              thus
                                consequently         accordingly
                                as a result             then

       Purpose         in order to
                             to this (that) end
                             so that

       Emphasis     indeed                   surely
                            undoubtedly           of course
                            in fact                   certainly
                            clearly

       Concession     to be sure
                               though it may be true that
                               although . . . still
 

      Of course, not all sentences need to be connected with transitions.  Use them to make your prose and your ideas flow smoothly.  Remember also to vary the position of your transitions within the sentence; in other words, do not always put them at the beginning.