From the rules: "If you see a good and correct answer, vote it up. If you see an incorrect answer, vote it down."
That's a very easy way to look at the point system, but what makes a answer correct/incorrect or good/bad? The first point is of course that your post is correct. If the answer solves the askers problem this have been fulfilled. But bare in mind that there are grades of correctness. A moveset need EVs, Nature, Moves, etc to be considered good, not just the Pokemon and 4 moves.
At the same time you don't want to bog your answer down with side-information. This is about focus, keeping your post fixed on the essentials of the question.
The second aspect is your sources. Generally your posts need a source to be considered valid. If the question is common knowledge or lacks a clear source (such as "only Game Freak knows" questions), a source might be impossible to find, but in any case where there should be a source, make sure to include it in your post as proof that your answer is correct.
When the answer isn't clear, or is of a more subjective nature, correctness only get you so far, you also need logic. This might be a strange thing in a world as illogical as ours, but if you lack sources logical arguments can serve as near perfect substitutes.
Of minor importance is speed/time. Answering first should not be your first priority, a well done answer is always better than a rushed one, but spending a day or two is a bit to long. And the same goes for really old answers (unless you have substantial improvements don't answer old questions).
Then we have the structure of the post itself. Good formatting, proper grammar, correct use of punctuation and upper/lower case letters will help you a long way. This isn't necessary, but highly recommend as a grammatically correct answer will be much easier to read/easier to understand. Also try to keep it short, no one likes reading through giant pages for something they that can be explained in three lines.
This basically means an answer here is writing a good text, one with balance between facts, sources, logic and structure. What it boils down to is effort, taking a bit of time to help someone who needs it. You don't have to put your entire heart and soul into each and every question but perhaps but a kidney or a spleen in every second or so.
If you have any questions or concerns, particularly with sources and formatting, that's more or less why we Experts exist, so don't be afraid to order us around a little.
Examples:
http://pokemondb.net/pokebase/249138/a-little-clarification-about-sliggoos-evolution
A clear question with clear and understandable answers
http://pokemondb.net/pokebase/4863/what-is-a-good-sharpedo-moveset?show=4863#q4863
Good formatting (generally) with good explanations.
http://pokemondb.net/pokebase/229135/humans-in-the-pokemon-mystery-dungeon-world
Good use of pure logic and post-game speculations (and shameless plugging).
Focus/Grammar/Simpleness can be found in each of these answers.
