Which is wicked. The little word can ensnare your idea in a tangle of subordinate clauses:

Dracula's castle, which is surrounded by a craggy landscape, is the scene of Harker's first encounter with horror, which makes him doubt his own sanity.

Which forces you to put words beside one another instead of making them move. Getting ride of it helps the sentence illustrate, not just mention, its point:

Dracula's castle, surrounded by a craggy landscape, is where Harker first encounters horror and comes to doubt his own sanity.

Try to turn which-clauses into adjective-clauses:

The landscape, which is craggy and inhospitable...

The craggy, inhospitable landscape...

Try to allow only one which per sentence, and look critically at final clauses beginning with which. These clauses often sound like afterthoughts:

Mina Harker is the real hero of Dracula because she uses her mind more fully than the men do, which makes her powerful and purposeful.

Mina Harker is the real hero of Dracula; she uses her mind with more power and purpose than do the men.

You can use either which or that for relative clauses modifying nouns in the sentence (who, whose, whoever, and whichever also function this way):

The book which I left on the table...

The book that I left on the table...

But only which can introduce non-restrictive clauses that function as asides or afterthoughts. That belongs to restrictive clauses that help define or situate the noun:

The book, which I left on the table, scared Aunt Nellie out of her wits.

The book that I left on the table was missing when I looked for it.

Note an exception to the "one which per sentence" rule in sentences with long parallel phrases. In such sentences the commas reinforce the parallel structure:

Dracula works a kind of evil which originates in a hunger for power, which ensnares people at their weakest point, and to which his victims eventually give up their souls.

Of, like which, can turn a sentence into a dizzy mess:

Evil is the betrayal of the condition of the love of fellow humans.

By turning an abstract noun into a verb, the writer gives more energy to the idea and allows room for further development:

Evil betrays human love when it seeks its own power.

Evil betrays human love because it replaces love with power.

A good rule about it comes from Sheridan Baker's The Practical Stylist: "Cut every it not referring to something." Substitute a noun of it at the beginning of a sentence:

It is hairy palms that repulse me.

The hairy palms repulse me.

After the party, the place looked like a wreck. It really bothered me, so I started to clean it up.

After the party, the place looked like a wreck. The mess really bothered me, so I started to clean it up. (The second it refers to the mess.)

   Sentence Fragments
   Comma Splices
   Run-on Sentences
   Dangling or Misplaced Modifiers
   Subject-Verb Agreement
   Split Infinitives
   Which, That, It, and Of
   Semi-colons and Colons
   Apostrophes
   His and Her