I have never entered or judged a competition, but reading recipes, studying styles and results from HB competitions, I believe I have observed that pushing the ABV of the style seems to yield more favorable results. If others feel that this may be a perception bias, then feel free to disagree and teach me to the contrary.
And that's not to say you can brew a 9% mild ale and win... Judges would knock that for being out of the style guidelines, but pushing it to the edges would be my strategy. I mean, let's face it, what homebrewer doesn't like a strong beer? It will make the competitors seem watery by comparison.
Obviously, this suggestion probably wouldn't work in the styles where lightness of flavor is important (American Lagers, Berliner Weiss, Hefeweizen, etc).