Be honest. Don't try to be someone else or something you are not. A lot of people get caught up in imitating other writers and they forget that writing is about YOU and who you are, your experiences and your life. Poetry is not about sounding "pretty" or "good", but about being honest and frank. Sometimes it works just to say it, straight out, what you want to say, and from there add the details and the insights.
One thing I have learned which really helps with poetry is be specific. Avoid words like love, hope, war, hate, pain, joy, etc. These are huge, broad terms and it is hard to relate to them. You can say "love", but it's much more powerful to say "love is two people lost in each others' eyes". "War" is not "people dying and killing" but the "splayed open insides of a young man on crimson grass".
Lastly- details work! But don't get lost in them. Some people get caught up in writing pretty words and pictures and forget about what they are really trying to say.
One thing I wanted to add- Someone above mentioned having a big vocabulary. Poetry is NOT about being smart or fancy. Any form of art is about expressing your experiences so that as many people as possible can gain your insights and relate to you. Using big words will only shrink your audience.