1. Don't go against the norm for the sake of going against the norm.
I know this happens. There isn't a film critic who would admit to doing it but I know that they do. Now, this is not the same as liking a film that got bad reviews, for example, I happen to have a soft spot for Scream 3, or disliking a film that got good reviews, I hate Born on the 4th of July. I'm talking about when someone sees that a film got a 98% and just to be a dick, he gives it a bad review.
2. DON'T ASSUME WHAT THE AUDIENCE WILL THINK!!!
I cannot tell you how many times I have seen this. A critic will review a sequel to a film with a huge following, such as Scream 4, and they will say "fans will be disappointed with this one." Well, how do you know? If you had said "I was a huge fan of the original but I disliked this one" that is something else entirely. But saying that you will know what an entire group will feel is just ridiculous.
3. Don't oversimplify or overthink the film.
If a critic goes into a mindless action film (such as the recent film Fast Five) expecting something on the level of The Terminator, he will be disappointed. look at a film for what it was made to be.
4. No big self-congratulatory words.
The purpose of a movie review is to tell the audience what you thought of the film. If you use very complicated words to get your point across, it isn't a movie review now it's the critic saying "ooh look how smart I am I can use big words." By using big words, the critic generally comes across as high and mighty and smug.
5. Go into detail.
Just writing "I didn't like the acting" isn't going to cut it. Go into detail. say something like "The acting came across to me as flat. I got a sense that the actors didn't care about the roles and it showed in their performances. The acting generally went from underacting to extreme overacting during the 'serious' moments of the film." With the second one, I fully understand why the critic didn't like the acting. Wit the first one, I have no clue why the critic disliked the acting.