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So it's all about the first line here, as in many other forms of writing. Use it, in a narrative-type scene, to either set the scene or establish reader interest. A good trick is to start with dialogue, as it immediately and naturally brings questions - the answers, carefully placed, can be used to set the pace of the piece.

The first paragraph cannot dominate the piece - a common temptation is to over-describe the scene, but the character count is an unforgiving master. Be utilitarian (I like to focus on details). Drop a few subtle nudges if you must. It's a sketch, not a painting.

Pace and rhythm are the dominating consideration, generally - whenever I can, I use triple repetition and/or a repeating motif to add impact and coherence. Resist the temptation to over-achieve: interest, development and suitable resolution all must be present in the same piece, and it is a real challenge.

Oh, and find a good last line. The ending is as important as the beginning, and more easily remembered.