I firmly believe the key to not choking is focus, pouring all of your focus into the match, because the only way a person chokes is by psyching themselves out and focusing on the fact they are in tournament or focusing on whats at stake if they win/lose. Redirect your focus to "I'm playing a match" and you will see a big difference in your quality of play, you won't instantly solve your choking problem but it will be a strong start.
Though things to focus on are needed in order to shift from "Oh god its a tournament match" to "focus on the match" , Once you get a grab you focus on their DI, when they roll you focus on which way they have a habit of rolling, focus on what they will do out of shield etc. Their are TONS of things in a melee match to focus on and as times goes on you will find more and more things to distract you from the stakes of a tournament match and it will become just another match to you.
For me what helps greatly is remembering all the combo's I do in practice and then once I see an opportunity to pull it off I say to myself "Ok, just like practice, this, this this and this...ok off stage, go into guard mode." Its a process that doesn't allow for any down time. I.E. I never have the opportunity to go "Oh god I am in a tournament match." Because I am thinking about several other things within the match.
Another way to think about this, if you don't want to go and identify a million things to think about is just to not think about the match as a whole and try to think about smaller bits and parts of the match, thinking "I want to win" won't get you a win but thinking "I need to get a grab/launcher/guard" will get you the win, and as I have said it will most likely take your focus off of the negative psych your self out thoughts.
TL;DR - Focusing on the end goal will not get you there, focusing on traversing the path to the end goal will.