I agree about balance - I also think the focus of winning winning winning suggests the subconscious has a relatively strong focus on (not) loosing - keeping in mind "not" doesn't compute.

Positive thinking is something lots of people talk about and I do think it is important - Boom Boom Mancini had a single negative thought in training and he called his mental coach right away (according to James E Leohr (spel?) author of a couple great books (Toughness Training For Athletes and The Power of Full Engagement)

There is an exercise I do with all beginners

Hold arm out to side and flex and hold as strong as possible. A partner slowly tries to bend the arm at the elbow(the goal of the 'bender' is not in the bending but seeing how much strength it takes to bend).

Second time the arm goes out straight and the student is instructed to hold the arm relaxed yet keep it there (loosely point to a spot on wall can help) and the 'bender' again slowly adds pressure to see how much strength it takes to bend.

It is amazing how strong the arm can be when remaining relaxed - I recommend anyone to experience it.

The point is and how it is related, I feel, is that the mental state you are in will support the body being tight and rigid and straining in strength or to remain loose and fluid and relaxed while still being very strong and powerful in your movements.

When you actually feel the strength difference and can have a conversation while someone is trying to bend your arm you get some solid feed back at what many of the greats tap into often yrs into training and competing (Thais start pretty relaxed generally I think)

Another one is to hold arms out, ground, relax and do like a dead weight - two people lift you by upper arms...

Compare how easy it is to lift when you are tight -

"I am the best and I will win", anger, stress, tightness etc and the body is much easier moved or lifted....

That is a physiological response to the mental game never mind the performance increase when the mind and body are relaxed

in my opinion and experience

(I do with beginners cause I find it helps loads (just like other stuff talked about above on this thread) and if not done from get go I find it hard to get across to people - say fighters with 10 fights or a few yrs of training