I don't like competitions now but when I entered competitions when I was younger I won almost all of them. Most important is mindset. My mind was set to win and to be better than everyone else. This confidence encouraged me to work harder than any of the other competitors (not that I knew that for certain but I worked very hard so I was satisfied I put maximum work effort into the competition project). Because I felt I was better prepared and presented music which was much more advanced than most of them (or an better interpretation of a required piece) this showed confidence in my stage presence and mindset while I was playing.
If you just think "I'll give it a go it will be a good experience to be in a competition" then do not expect to win or even come a place. It will in fact reduce your chances of doing well in a competition if you take that passive mental attitude of "not caring" about the result and just wanting to have fun. Competitions are not a place to have fun with your music, there are better places for that, competitions are brutal and unfriendly in reality, every man/woman for themselves!
Record yourself all the time, the entire program you will present without stopping then listen to the results. There will always be imperfections and your aim is to produce a recording which is as close to perfect as you can get. The more times you produce these type of recordings in a row the more chance you will produce the same in competition or exam even.
Non musical presentation is also a detail that you should think about. Do not over dress, try not to wear the cliche black pants and white shirt with bow tie. Smile and show your teeth when you smile as you walk onto stage and off. You might not feel that it is very important but it will effect the jury members. They will sense that you are at ease, present a friendly aura, makes you more likable somewhat. It is better than a stone faced sad sac. Do not strut onto stage with a puffed chest and full of arrogant confidence, but at the same time do not slouch and hunch over as you walk onto stage try to be normal! A smile is very important however please provide your audience and jury members with a couple! You should practice this before hand if it doesn't come naturally
