Try exchanging all your pieces if your up a piece / pawn(s), to win the endgame easy.
How can I win a clear advantege game

I've had a game where I became a queen up on my opponent but lost anyway. I think it was just due to complancency because since I had an extra queen I wasn't concentrating or trying as hard. True, the checkmate was due to purely positional reasons and not because of a material advantage, but it's easy to have a lack of concentration when you think the game is in the bag early.
I guess it is a strange question..
But I noticed while I play fast chess games, I have many times a clear advantege or winning positions ( like i have 2 pawns, a piece or a general advantege in matrial .. etc ) and i loose these games. due to time pressure may be, or may be i feel lazy to think as i did after i got a winning position because i expect the opponent player to resign..
please advice me. what can i do.. it sounds like getting a clear advantege/winning position is something, and winning the game is another game.