The phrase servirsi da, which literally means "to serve oneself" refers to the concept of helping oneself: Grazie lo stesso; ci serviamo da soli. (Thank you anyway; we'll serve (help) ourselves.)
The phrase servirsi di means "to use" or "to avail oneself of": Per adesso mi servo di questi libri. (For now I use these books)
And finally, the phrase non servire a nulla means "to be of no use": Queste cose non servono a nulla. (These things are useless.)
