What devices does Viola use to get into the Duke's service?

Viola uses disguise to going into the Duke's service as a eunuch.

“Conceal me what I am.”

This shows Viola's practicality, although we're not yet quite sure what she's planning. Viola also knows that money talks and that with money she can accomplish most anything; money represents power. Viola's payment to the Captain is another act of practicality. She knows what she wants, and she does whatever is necessary to get it.