Soooo
Here's what this really is:
https://en.wikipedia.org/wiki/Flow_(psychology)
This should be helpful:
Let me explain in terms of Smash Bros. Let's go down the mindset of what you need to do is win.
1) You need to win.
2) You need to know how to win. Knowing the match-up helps. If you don't know the match-up, know what you need to do in order to learn the match-up on the spot.
3) This should be pretty clear. Just know rough kill percents. Jiggs doesn't die at the same time as King Dedede. Etc.
4) Does not apply
5) You're not going to get in the zone if you think you're fighting some scrub. Understand that each person has a chance to beat you. Even if you have fought them before, they may change their playstyle in a way that ****s you over. Being careless is what will lead to your defeat. Give it everything you have, because they are all threats. Or, if the person is truly really bad and no challenge, and you still want to be in the zone, change your goal. Will talk about other goals later.
6) Don't go off thinking you're just some scrub either. Just like they have the ability to beat you, you have the ability to beat them. Think of what you have done in this game. If you do lose, it is not necessarily skill. Match-ups are a thing, and even beyond that, playstyles can have an affect. A person who is shield happy will be at a disadvantage against somebody who is grab happy. It does not mean you suck.
7) This is why you bring the headphones. Read this (it's easy to understand):
If you are in a tournament, there is going to be a lot of people behind you. A lot of talking. Other sets are going to be going on too. There is a lot of information to pick up on. You might be able to 'tune that out,' but eventually, something is going to happen which causes somebody in the crowd to yell, or say your name, or your character's name. What music does is block those possible distractions by having something else to focus on. You only get the information from the song, not the random people. Furthermore, if the song happens to be one which you know well, you will not gain much information from it at all, and your focus can be placed onto the match. I recommend a song which also makes you feel powerful and has a nice beat, because it will more trigger your sympathetic nervous system, which will improve your reaction time. However, be warned, that if you are quite INTROVERTED, you do not want to have the music very loud. However, you might need to have the music louder in order to block people out, unless you have the right headphones. This is why I do not recommend earbuds, for they do not block out anything. The sounds still vibrate the part of the ears which are not covered, and the vibrations will make it to your eardrums. Better headphones for focus are closed, semi-closed, or semi-open (AKA noise cancelling or semi-noise cancelling).
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Now then,
Basically, in either situation, you need to experiment. If it's too easy, try some things out that have been on your mind. If it's too hard, look for different paths to winning.
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Anyways, according to this page, some other things which help with flow is internal, intrinsic motivation. You could possibly change that first step from winning to:
1) Learning a match-up
2) etc
Or
1) Having a good time
Or
1) Developing the character's technical skills
Or
1) Testing/working on combos
Or
1) Demonstrating your skill as a player
The list goes on and on. You do not necessarily need to make your aim winning.
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One final note is if you do lots of online play, I suggest you change all of the music on every stage to be songs which you find are inspirational and allow you to be in the zone. It is a good idea to have your headphones plugged into the gamepad rather than listening to things on your ipod/mp3, because you actually react to sounds faster than you react to visual. You can react faster to hearing the opponent's attack than seeing it. To have both the zone and this reaction time boost is a great thing.
Hope this helped!