rest on one's laurels
Definition: to feel so satisfied with one's achievements that one does not make any more effort to improve
Example: If you still want to be the champion in the next competition, don't rest on your laurels.
Etymology: based on the fact that winners of competitions in ancient times were often awarded a ring of laurel leaves (a laurel wreath) to wear on the head as a sign of victory
Usage note: derogatory idiom
See also: R Idioms, Verbal Idioms