The Harley Owners Group (HOG) has a program called Fly 'n Ride. The cost for a U.S. rental is very reasonable at $100/day with a 2 day minimum. This program is available to HOG members only and the motorcycles are picked up/returned at participating H-D dealerships. Reservations must be made directly through the HOG office in Milwaukee.

Individual H-D dealerships have recently jumped into the rental business without affiliation to the Fly 'n Ride program. They are relatively expensive compared to the HOG program but can sometimes offer good deals for return/repeat renters. Don't be afraid to ask.

Eaglerider is a national company that rents late model Harley-Davidson and Buell motorcycles by the day. Of course, their rates are cheaper the longer the rental period. I've rented from them in Las Vegas & Denver a few times and had good experiences. They have an excellent website that makes online reservations a breeze. 

Google : Motorcycle Rentals

Every place I've rented charges a fairly high, unavoidable daily insurance rate that can run as much as $25/day. Renting a motorcycle is not like a car rental wherein you can choose to avoid their insurance coverage because your credit card company car rentals. Feel free to call your cc company and ask but I doubt you'll get a positive answer. Also, most rental companies will demand to put a temporary $1-2k charge on your credit card as a security deposit. This is a common practice.

Eaglerider provides helmets, raingear and a bike lock as part of the rental cost (though I always bring my own helmet) for both rider & passenger if applicable. HOG's website states you must provide your own helmet(s).