First and foremost: your job as a reviewer is to entertain and/or enrich. A review, a proper review, is not a blog entry. If you're writing it and posting it publicly there is a presumption that it will be of use to others. So if something gets in the way of that, if you have to choose between doing something you think a review is supposed to do, versus saying something interesting...say something interesting.
I used to write reviews in a checklist fashion. I'd open with some angle that struck me (which is good) and say all the things that I found most compelling (also good)...and then I'd read it back and realize I didn't mention the quality of the acting or the photography or whatever, and feel like I had to put in some perfunctory mention of those things (which is bad).
Your job is to hold the reader's attention for however long you're asking for it, and to say something about the film that will be amusing or insightful or <insert positive adjective here>.
To that end, the only really hard, specific piece of advice I'd want to give was already given right here:
One thing that I don't enjoy in movie reviews is when they take up a ton of space giving a plot summary or a blow-by-blow account of the events in the film or just other stats that are just basically IMDb information. I'm not reading a movie review for a book report, I'm reading it to enjoy it as a piece of writing.
Huge co-sign here.
Sometimes you need to reference the plot so that the review makes sense in isolation, without having seen the film, but it should be quick and efficient and only do what is required to achieve that.
We have guidelines for what we'll tag as reviews here, and one of the guidelines is specifically that the review cannot consist largely of plot description.