Forget everything that happened before...
... except how to play good, solid poker.
If I've just cashed in a tourney, I tend to lay off any more poker until the next day, feeling "all pokered out". (Well, in reality, at least the next couple of hours!!!)
Walk away, take a break. You've earned it! It's hard to be patient for three to four hours and THEN try to do it again!!!
It is a huge ego boost to finish ITM. Don't let your ego rule your next tourney.
Forget the premium hands you were dealt.
Forget the 'bad beats' you put on others, and the suck outs too.
Forget when you were forced to play a less than premium hand from the SB/BB, but still won the pot.
That was then... this is now. And having done it once does not guarantee the ability to do it again.
Play every tourney as if it were your last $ on the line, not just 5% of your BR.
DO Remember that it just took you up to four hours to win, and only seconds to lose, when playing the wrong hand, or indeed, at the wrong level of play.
Make sure your bankroll can withstand the buy-in. I mean, what's the point in sweating over the PC for hours on end to win $20 in a $1 tourney, just to lose it all in a single hand, in a tourney with a $20 buy in?
I've done it myself, so don't think I'm being too critical. These are just pointers that work for me.