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Non ministrari, sed ministrare (VG Mt 20,28)
Is a well-attested phrase with that exact meaning. It literally means not to be served but to serve. The context is Jesus in the Gospel of Matthew saying that He (the Son of man) came not to be served but to serve.
Update: it is (arguably) a common choice for mottos. Besides the American College of Greece (see comments), it is also the motto of Wellesley College in MA, USA (among others) and at least six recent catholic bishops.