Always look for tactics.At every move!First look for the most "impossible" move.You might be surprised at how often you can sac a piece for a pawn and regain it a few moves later coming out a pawn or two up,maybe even a piece.Especially look for capturing a defended minor piece with a rook or queen,those get overlooked the most.Keep an eye out for knight forks!
If your position is worse try to avoid trading pieces and instead mix things up,make it difficult in an attempt to drown your opponent in the tactical swamp.Of course that cuts both ways but hey,your position was worse anyway
In the endgame if you're down pawn(s) trade pawns and keep the pieces on the board.If you're up pawn(s) trade pieces not pawns.
In general keep in mind people are sloppy,especially when they're up material and/or think they have the better position and are easily winning.There lies your chance.Don't be afraid to sacrifice.
Go for glory or die trying!