This week, as you revise, follow the advice in Strunk and White's Elements of Style, and go "which-hunting." That is, look for places you have used the words which, who, or that to introduce a subordinate clause, and see if you can eliminate the need for that clause.
Count the chairs which are in the room
can be revised to
Count the chairs in the room.
Not only have we eliminated two words, we have kept the reader's brain from having to process a whole extra clause.
As Dorothy found out, not all whiches are bad. But watching for them is still a good idea.