Between one year-end closet cleaning and another, I ran into a little bullet-point list I wrote up a while back, partly for myself and partly for other folks. (Full article after the jump, for the sake of those skimming. I have to remember to use that feature more often.)
Use the research others have done. If someone else has written a good plot summary, read it and use it to get that much more perspective on what you've done. (Obviously, plagiarism is a bad idea.) Cast lists, crew rundowns — odds are someone else has already done it. Get yourself up to speed before you throw yourself in and start swimming.
Leave subtitles on. One of the reasons this is useful: you can grab screenshots of crucial lines of dialogue, instead of having to write them down as you go. Even if you're watching in English, this is still terrifically handy. (Note if you're using VLC: you can temporarily remove subtitles from the picture — for the sake of a screenshot — by hitting Pause, then Shift-Left arrow to backtrack, Shift-Right arrow to move forward, then frame-stepping forward to get back to the frame you were at, as you might be a couple of frames off. It sounds more complicated than it is. Honest.)
Take notes. This goes hand-in-hand with leaving titles on. You don't have to write a whole dissertation in the other window while the show's still unspooling, but pause long enough (or take breaks at the ends of episodes) to jot down the things that seem to matter. Full plot breakdowns are not required; most people reading don't look for Cliffs Notes rundowns of storylines, anyway.
Watch in big bites. Don't watch just one episode at a time; watch as much of a disc as possible in one sitting. You get more of a sense of how the show progresses and evolves this way, and you can compact a lot into less time. When you skip opening and closing themes and "Next Episode" bumpers, you'd be surprised at how much faster a show can speed by.
Cap often. Take screencaps as you go. If you see a shot that jumps out at you, don't wait to go back and look for it again; grab it now. Disk space is cheap. You can always delete the pictures that don't suit your needs, or archive them offline somewhere.
Develop a style guide. This is more for the sake of your own internal consistency than anything else. Keep a uniform style for quoted material, titles, external links, etc. It makes your work look all the more professional, even amongst people who might not consciously look for such touches.
Admit your reactions may be all too personal. A critic named Robert Warshow once said, "A man reviews a movie, and a critic must be willing to admit he is that man." Meaning if you don't like something, you have to be willing to swallow that and put it aside for the sake of asking yourself okay, will someone else like it? I choked down my aversion to moé titles long enough to review Kanon, but one exposure to that sort of thing was enough to convince me other people would be better equipped to deal with it. At the very least I was not going to allow myself to use the review as an excuse to let fly at moé. That's what blogs are for.
Know the target audience. There always is one. If you're reviewing a show you're probably not the audience for (e.g., me and Kanon), think about who the target audience is and what they want out of it. Some people think Kanon is heartwarming and sweet, and think Ghost in the Shell: Stand Alone Complex is just plot on top of more plot all the way down. Walk around in their shoes for a bit. Not for the sake of changing your own mind, but for the sake of speaking to others that much more effectively.
Get out of your comfort zone every so often. Do yourself the favor of watching things that you'd normally not seek out on your own. Example: Kanon, again. Even if I was fairly sure what to expect from that show, I still took on review duties for it. Ditto Saiunkoku, which I thought was going to be fluff and turned out to be epic.
Read your work out loud. Yes, even if you hate the sound of your own voice. It's easier to get away with something bad on paper than it is when you're talking to someone face to face. Do this and you'll probably discover you have at least two or three verbal habits that strike you as just plain wrong (because you'd cringe if they came from someone else's mouth), and you'll move to do something about them.
Do not be afraid to dissent. Note that "dissent" does not mean "my opinion right or wrong." If you honestly think Bleach is a bore or that Naruto should have been put out of its misery a long time ago, feel free to say so — but defend yourself. Tell us why. Back it up. Go to the barricades for your point of view. Stick your neck out. Don't simply stamp your feet and expect others to follow. If you really feel a given show is underrated — or overrated — put that feeling into words, and back it up. It's through such things that you find your voice, and your tastes, all the more.
Don't let any of the above stop you from enjoying, or not enjoying, anything you watch. You should never be ashamed of having a good time, no matter what you're watching. Convince someone else they can have a good time with it, too, and you've made that much more of a difference.