This discussion has always interested me because some people think they can be defined, some people don't, some people hate to think that just because somebody thinks more abstractly or catches on quicker but can't waveshine perfectly can still win, and some people like that idea... the differing opinions across the board make for fun debate.
I, however, have always maintained this idea; think of the number of techniques, strategies, combos, "mindgames" if you will, as words in your vocabulary. Maybe you'll go look up a new "word," learn how to use it, try to incorporate it in your day to day usage... maybe you'll forget a word, hear it again, and think "man, that was a cool word," and try using it some more. Eventually, with enough practice and study, you know so many words and know how to use them that you can string together esoteric sentences so you become the equivalent of William Faulkner or Jmes Joyce and nobody can understand you...
Now translate that into Smash. The more stuff you know how to do, the more "words" you have. Your "sentences" become more complex, when you want to do something you can find different words for it. If you're skilled at finding just the right technique for a situation, you can just pull stuff out of nowhere and it just fits... mindgames is simply the amalgamation of techniques you know and your ability to adapt them into different situations.... but then, the only wrench in this idea is the ultimate curveball... your opponent.
He's arguing with you! He's got a bunch of words, some of them you've seen before, some you haven't... and you have to figure out what the hell he's "saying" before he crushes you. Maybe he only uses one word... over and over again... maybe it's two. "Roll" and "fsmash..." very popular words, used quite frequently but perhaps too frequently... anyhow, Smash turns into a debate between you and your opponent. Person with the better argument wins... now you may know more words, but can you piece them together? Can you counter your opponent's? What if he says something you don't expect? Are you ready to respond?
Back to Smash. Let's pretend I'm on the ledge with falco... being edge guarded. I can think of, off the top of my head, seven "words:" dropping down, wall jumping away to forward b is one "word.." another is simply pressing L, or R and rolling past. There's attack from the ledge, jump straight from the ledge, drop down and jump up with a spike, simply press up, or reflector in place to slowly fall before jumping up to the ledge to psyche them out. Maybe with some more time I could get creative. But I don't have time, I'm being f-smashed by Marth, and his sword reaches over the stage. Well... I have to decide. But the more options I have, the more I can surprise him. The better a chance I have of catching him flat-footed. The better I am at varying my words, the better I will be at surprising him. The better I am at predicting his argument, the better I will be at choosing mine. Simply put, you need to have lots of options, know when to use them, and be ready to use all of them, whenever, wherever. That is the first part of mindgames... simply figuring out what to do, when, in response to what. The second part is figuring out what your opponent will do. It's anticipation, and it's adaptation, and it's recognition and performance all rolled into one.
That was a long read, probably, but there's more yet. For the basic player, what techniques should he learn? Most basic is simple mix-ups. Don't just roll back and forth and forward smash... not only is it irritating, it's stupid. Jump a little! Jump a lot! Run away, run back again, try a projectile instead of your tried and true c-stick.
Time passes and you're a better player, and now you can do so much more! Dash dance into a jump cancelled grab! Spot dodge instead, and grab again! Shield grab! Throw grabs out the window and wavedash backwards to forward smash. Backwards to forward wavedash to down smash. To up tilt. To a short hopped up air with fox for the k.o. Look at all the things you can do! Don't just do the same strategies because they "work..." do ones that don't work. They'll make your better ones that much more powerful for being unexpected.
Have you ever watched pro videos and wondered why sometimes they fall into such "obvious" traps, or they do stuff that seems random and pointless, or they jump around for what seems like hours before ever even attacking? They're feinting and expecting things that don't happen... it's like a game of chess where a player loses before a piece is ever captured. Most people shy away from using just "up" to get up from the ledge... why? Because you aren't _doing_ anything... but you don't have to! Doing nothing is a mind-game in itself! Some guy is sitting there with his shield up because every **** time you hit the "a" button from the ledge and he shield grabs it. Just hit up on your control stick, get over that ledge, and grab him! SURPRISE!!!! Somebody said in another post to never walk when you can run or short hop. Why not? I watched the MOAST3 finals and Ken walks, yes walks, away from Isai getting up from the ledge and then turns and kills him with a forward smash. Never ever ever say never, because each no is just one more way to lose.
Smash, for all its simplicity, is replete with myriads of options for you to choose from. The best players don't forget any of them. People get better by learning and using more of them, and they get worse by forgetting. So go ahead, post away your favorite techniques and responses to time-honored ones. Someone's got a question about shield grabbers? Tell him all the things he can do! Now he knows, and knowing is FAR more than half the battle. You can dash dance to a grab, wd to a smash, land behind them and punish their throw with a dsmash, or just stay on the other side of the level! Even better, humiliate them by walking, yes walking towards them, and staying out of range of the throw and busting their shield apart. you can do so much, so don't say, "there's nothing I can do," think "what haven't I tried?"
Sorry that was so long. I got carried away.