Hmm, well I find one of the best ways to do it is just write whatever is in my head.
Then, afterwards, I will play around with word order and line/stanza order to make it sound interesting and unique.
Then I do a final read and see how it sounds... And if I don't like it, I never scrap it, I always just try and make changes.
But you just need to be spontaneous I guess. It's all about the feelings and thoughts, and if you can get those down fast enough then it will usually just work itself out, whereas if you try to think about it too much it will come out bad.
That's just my opinion though.