A lot of new entrants don't pay enough attention to the style guide. So, make sure you enter your beers in the correct style category. Taste your beer while reading the style guideline for the style you are planning to enter. Does it have those qualities? You might set out to brew an American Amber Ale but the finished product tastes more like a pale ale or an ESB. Enter it as what you think it ended up as versus what you originally set out to brew.
Another question I ask new entrants is did you add anything to the beer so that it would need to go into a specialty category like spices, chocolate, fruit, wood aging, etc. That's a big mistake I see a lot of brewers make.
Aside from that, RDWHAHB! Judging is not a perfect system, but it can be educational and rewarding most of the time. Here's hoping you get some good judges! (On that line, try to enter a competition that has a good reputation for judging and organization.)