In most cases if you begin a sentence with a this, that this should be followed by a noun that refers to exactly what it is that you want to point to. Following this advice will add coherence to your papers. Notice that I've just taken my own advice. Had I said "following this will add coherence" and not supplied the noun "advice" you might wonder what I was referring to. Often the noun that should follow this has already appeared in an earlier sentence, but as another part of speech. For example,
The morning was cold, and
Joe struggled for several minutes to start his car. This struggle
was rewarded
when the engine finally
growled its way toward life.
Notice that the verb struggled in the first sentence becomes a noun in sentence two, creating the necessary link. Whether a given collection of sentences appears coherent often depends on these small but important touches. Sometimes we can avoid supplying these links in conversation ("Joe spilled coffee on his wife's dress. This made her angry."), but formal writing usually benefits from them. Student writer will sometimes fail to supply needed links because they fear repeating themselves, but careful repetition is what, in fact, builds coherence.
There is probably nothing that will improve your writing more with less effort than taking care of vague "this" problems!
"It is" or "there is" openings: Beginning sentences with "it is" or "there is" invites wordiness. " It is Freud's argument that civilization is a mixed blessing" can easily be shortened to "Freud argues that civilization is a mixed blessing" by changing the noun "argument" to the verb "argues" and eliminating the "it is." Be alert to opportunities for conciseness. "It is important to count every vote" is reasonably concise, but even this sentence can be shortened to "Every vote should be counted."