Sam Allardyce remained cautious despite seeing his West Ham team take a 2-0 advantage in their play-off semi-final first-leg with Cardiff.
Two first-half goals from Jack Collison secured a victory for the Hammers, with the return leg at Upton Park on Monday.
"We're not going to get carried away. It was a terrific result but there's a long way to go," Allardyce said.
"We need to be as professional on Monday as we were in Cardiff and that should be enough to take us through."
He added: "Two goals is a margin that's difficult for them but they'll be thinking about getting the first goal and whether we'll be nervous and edgy and trying to protect what we've got."
Allardyce has experienced a similar advantage in the past, having guided his Blackpool team to a 2-0 win in a play-off semi-final first leg at Bradford in 1996, only to suffer a 3-0 home defeat and lose his job.
Seeking an immediate return to the Premier League following their relegation last season, the Hammers were overwhelming favourites for automatic promotion.
Allardyce feels that his players are now playing with a new sense of freedom, after spending recent weeks attempting to catch Reading and Southampton, who finished in the top two.
But he thinks it will be vital to attack Malky Mackay's side on Monday, rather than sitting on the two-goal cushion.
Allardyce added: "The last three games we were relying on other people even if we won and it's in our hands again now.
"Whoever wins, takes the trophy and it's not about us playing catch-up on anyone, we're just trying to deliver a performance and we've got to make sure we finish it off.
"In the back of the minds there might be a slight hesitancy to try to protect what we've got but that would be dangerous and we've got to have a positive attitude and try to score the first goal.
"It's my job to start mentally preparing them for the fact that they've got to approach the game as a 0-0 and they've to try to win it."