It's almost a catch 22 with homebrewing competitions. The primary objective is to create the perfect example of the style in which you enter, yet your beer has to stand out against the others entered into the same category or you risk going unnoticed.
This is where realizing the difference between round one and round two can be valuable. Both rounds are important. You have first to survive Round 1, but Round 2 is where the real wow factor may come into play.
During round one, your beer will be placed in a "Flight" with 6 other beers in that same category. Each will be judged individually, with the score sheet (see BJCP.org) where the notes and numbers will be tallied. Several sub-categories may be in the same flight. For example, in Category 18, Belgian Strong ale, the flight may consist of any of the 5 styles in the Category. Your Dubbel will be judged as a Dubbel, but a Tripel or two may be in the same flight, being judged as a Tripel. At the end of round one, at the option of the judges 1 or more of the beers will be "pushed" to round two. Generally it is the beer (or beers) with the highest scores. If you make the perfect Robust Porter (and it is entered in the correct category) you are likely to score well and be "pushed" to the second round. For on the process and scoring for BJCP Competitions, see also this previous post on the topic.
During the second round, the "pushed" beers from the Category are lined up, and the judges sample each and talk about them. The judged decide which of these beers will take first place of the group, then second place, etc. Usually, the ones to eliminate from consideration are selected first.
This is where that perfect Robust Porter needs to be not only very perfect, but interesting. If there are two perfect porters, which will the judges select? This is VERY subjective and what they think is what will happen. I've heard advice to create you beer so it is on the peripherals of the style guidelines. Make it toward the high end of the IBU scale or ABV scale for the Category. I think this makes sense. And depending on the Category, you may want to be toward the low end of the scale. But it has to be within the guidelines, drinkable, and memorable. Simple, right?
A simple idea... Read the guidelines, and if a descriptive word stands out to you, perhaps gear your recipe somewhat around that word or phrase... but stay inside the guidelines.
Something else to consider... As always, proper care with sanitation, fermenting temperatures, yeast pitch rates, etc. will help assure your beer turns out the best it can.
Good luck and happy brewing. Cheers!