I must admit that I have not won any big cosplay competitions (but I haven't entered any large ones either). I do know a number of people who have won big competitions, interviewed cosplayers who are also judges for a University project, and last year I interviewed Euro Cosplay contestants. So although I can't say this is 100% guaranteed way to win competitions, but is more of a guide put together using other people's experience as contestants, judges, and lovers of cosplay.
Depending on the competition the following points will have more weight than others. Some competitions focus more on impressive techniques, others on props, and there are often prizes for amazing sewing skills alone. A good bet is, if you can't apply all of them to your costume, use a mix of the following points.
Made By You
This is probably obvious but the costume needs to be made by you. No commissions, no items made by other people are allowed. A cosplay competition is to show off your own skills, not someone elses.
Recognisable Character
When the judges know what the character you're cosplaying is from the more likely it will stick out in their minds. But recognisable doesn't have to mean popular. For example, there are so many Frozen and Attack on Titan cosplays now that it's hard to pick any one from the crowd, and in a competition there is likely to be more than one of those entries. Avoid them because judges are less likely to pick out something they have seen 100 times before. But likewise a costume based on an original character might be impressive but it's less likely to stick out to someone compared to a character they recognise.
