Winning the won game.
Strangely, it's not easy to win a game of chess when one is in a winning position. There are a few reasons why this is so:
1) underestimating or allowing counterplay from one's opponnet
2) Your mind starts to become lazy and miss simple tactics
3) incorrectly evaluating the position as winning.
Hence it's always good to practice STOP during those critical movements in the game before making a move.
S- keep a steadfast mind, don't let your opponent back in the game
T- Is there any tactics in the position that I'm not aware of?
T also stands for Time management; have a glance at the clock and make sure you're not much worse off in time then your opponents
O- objectivity, break down the position into it's positional elements and find the best move from there.
positional elements include
a) pawn structure (kingside/queen pawn majority, passed pawn
c) king safety
d) minor pieces/major piece placement.
e) outpost squares
P- prophlyaxis, what's are your opponent main threat/plans? Do you have the initative? Do you have to deal with your opponent threats first?
The following game illustrates how easy it is to switch off when winning...