Hm.... I'm a little different. When an idea first pops into my head, it's usually a general concept. I work out the kinks of the concept. Then, I figure out the main points I want to get to, this includes a general idea of how I want it to end. I then start writing the story out, making changes to what I want to do including the ending. By the time I reach the end, the ending might be different then what I originally wanted to do!
As for tips, just think about what kind of emotion you want to achieve. Also, it needs to work with the story, of course. You can't have something coming from nowhere unless you plan to make a sequel.
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