My process for a Hilton I haven't stayed at is to e-mail the General Manager of the hotel and politely ask for an upgrade. I always state in the e-mail that I know it's not a guaranteed benefit, but that I just wanted to inquire on the possibility of an upgrade.

You can call the hotel and ask for the GM, or sometimes it's on reviews of the hotel at Trip Advisor. Once you see the name, the format of the e-mail is First Name.Last Name @hilton.com

I have a LOT of success with this strategy. It also works well once you have their e-mail for frequent stays at the same hotel. The key is to ask, not expect.