We all crave feedback. Yes, we write because we love to write, and because we love the movie, but reviews help keep us going, stroke our egos, and we want them.
So when do you review a story? That depends on your own personal choices, but for me, I review basically whenever I enjoyed a story. It was a few minutes of free entertainment, and I know how much I love getting reviews, so unless I'm in a rush or my computer/FFnet is having problems (one or the other happens frequently) I try to review. I forget sometimes (more often than I should) but it's worth it to try.
So what to say? If you're stuck for words, say what you liked. "I liked the tone of this story," or, "I like the way you write dialogue," or something like that. It's not the most expansive, mind-blowing review in the world, but I promise even that's appreciated. If you have more to say, obviously, say it. Long reviews and glowing praise make authors very, very happy.
Much harder to do is review if you liked a story, but also saw it's faults, or if you flat-out didn't like a story/found it to be bad. Let's start with that second one. The first thing to ask yourself is, does it warrant reviewing? That is, does the author seem like someone who would listen to a negative or critical review, and improve from it? In an ideal world, everyone would be able to handle criticism, but sadly, FFnet is not that utopia. Many authors won't care if you suggest things to correct; some authors actually react with anger and flaming to the suggestion that their stories aren't perfect. (Oddly, a lot of these authors are the ones who say "I know my story sucks just r&r!!" in summaries...)
So if you get the feeling the author won't care or won't react well, why bother? Honestly, I don't know. This is one of the few things I'm cynical about, but if I think it won't do any good, I don't take the time or the energy. I just don't read anything else by that author.
But if you suspect the author can and is willing to improve, than it's more than worth it to review with suggestions on what can be better. This is also goes for if you enjoyed a story but found faults in it that detracted from enjoyment.
Opinions differ on this, but my basic guideline is the old adage, "You catch more flies with honey than vinegar." That is to say, if you're nice and polite when you make suggestions, people are more likely to do react positively and do as you asked than if all you do is point out flaws. Maybe take a minute to try and figure out what was good about the storythat can be hard, I knowand state it first, something like, "I liked the way you wrote the conversation between Jack and Spot, but I really had trouble telling who was talking. It would be much easier to read if you used quotation marks." Or whatever.
Again, this is my method. I usually pick out what was best about a story, and what most needed to be improved, and give them both. If there was a lot that needed to be improved, I usually pick out just one or two things, because I like reading fanfiction and don't want to discourage people from writing it, which is what negative reviews can do. If it goes over well, I'll keep reading the story and offer more advice as time goes onif it doesn't, I write it off as something not worth further reading.
Giving feedback is a core part of a fandom; it turns FFnet from an archive into a community. Communities are great, so please, please, give feedback often. (It's also worth noting that people are more likely to review stories written by people who've written them reviews, and if you're looking for a way to start getting more, well, 'tis better to give than to recieve...)