Data performs his own maintenance
DATA [OC]: I am rarely in need of Doctor Beverly Crusher's professional services as my bio-mechanical maintenance programme is self sufficient.
The Star Trek Encylopedia explains the biomechanincal maintenance program as:
Software incorporated into Commander Data's positrnoic network. It kept him physically healthy and rarely in need of Dr. Crusher's professional services
A Fistful of Datas
DATA: And I will perform a self-diagnostic
As is later seen in that episode (A Fistful of Datas) we see that Data can connect to the computer which can perform a diagnostic on him also.
The quote from Data's day, however, does seem to indicate quite strongly that, unless something is seriously awry (as Xantec points out, was the case when Geordi performed a diagnostic on Data in *Clues), Data performs his own routine maintenance.
Because I anticipate someone asking, no Dr Soong did not perform maintenance on Data in Brothers; you will note he does the same thing with the teeth to Lore and straight after both instances, both Data and Lore are 'awakened' if you will, from their unconscious state.