The South Americans earned a 3-1 win over Croatia on Monday to seal a second-place finish behind Brazil in Group A.
However, Herrera has refused to get carried away by his side's achievement of getting through to the last 16 of the tournament.
"I'll have a few more great days yet," he told FIFA.com. "There's still room for improvement. We stayed calm and maintained our focus, that was the key to success."
Mexico will play Group B winners the Netherlands on Sunday for a place in the quarter-finals.