"We were successful in the European championship just over a year ago and followed that with a brilliant World Cup qualifying campaign," Del Bosque told reporters ahead of Saturday's friendly against Argentina in Madrid.
"What worries me most is the way the collective (team) functions, to maintain the excitement so there is no relaxation. Every day we need more effort, more desire, more ambition to win.
"A World Cup is coming which is exciting for everyone but we can't go thinking we are unbeatable," added Del Bosque.
Spain won all 10 qualifying matches and their only blip in recent months was the shock 2-0 defeat by United States in the semi-finals of the Confederations Cup in June.
Del Bosque said there was still work to be done on sharpening up the side's forward play and that Argentina would provide a particularly challenging test in a match to celebrate the Spanish football federation's centenary.
"Argentina have suffered a difficult qualification process but have come through stronger and head towards the World Cup as one of the favourites," he said.
"As we don't have many warmup games we need to make the best possible use of them."
With Fernando Torres out injured, David Villa is set to lead the line in Spain's favoured 4-1-4-1 formation, though the coach could also experiment with Alvaro Negredo playing at number nine and Villa pulling out to the left.
One player assured of his place is goalkeeper Iker Casillas who will make his 100th appearance for Spain.
"It will bring back many memories. More bad ones than good because of all the setbacks we have suffered in World Cups and European championships," said Casillas.
"But the one that will stick with me will be the victory in Switzerland and Austria (last year) which was spectacular."
