If you regularly rent from the same station, you might get to know the manager there. However, for such long rentals, they won't want to give you too nice of an upgrade b/c they won't be able to rent if for so long. One trick that used to work regularly: if I was renting for more than a week, I'd reserve a car for a day or two. When I got to the location, you can ask, "Since I'm only gone for a day, can you upgrade me to X (something nice I saw on the lot)?" Shortly into the rental, I'd call the 800# and change the return date. However, now that Avis is charging "late" fees regularly when you extend a rental, you might not wish to do this.

The only other option than being marked as a frequent renter in your profile (which you can't even see, but agents can as they pull your reservation), might be to get the Continental MasterCard or other card that gives you Avis President's Club status. That's good for a 2-class upgrade. Sometimes a location sucks, but on the whole, I found much better service from the PC membership than "Avis First." You must pay for these cards, though.

Edit:
It's also worth noting that if you're going to smaller locations (where you'll see the same employees again & again), little gifts work wonders! When I was Avis First/Preferred Select (before they changed the name), there was a run where I went to FWA and FSM every week or every other week for the spring & summer. After my second or third time, they each upgraded me to a newish SUV from a mid-side, at my request. When I returned the cars, I took back boxes of chocolates (<$10) and thank you cards. From then on, whenever I go to those locations, I can have my pick of the lot from the employees that I regularly see. I even got Christmas cards from FSM for a while, but not lately since I haven't been there in over 1.5 years. However, a few conversations at FSM went like this (when I only rented mid-size cars!):
-"Hello, Mr. IAH. The only SUV's on our lot are X, which you had last week, or Y which has a lot of miles. If you'd like, I can give you a convertible or Town Car, the biggest thing on our lot?"
-"Mr. IAH, HERE YOU GO, as they tossed my keys to me while I was waiting at the baggage claim. If you don't like it, just come back in."
-"We got a brand new Murano last week and I saw you were coming this week, so I specifically kept anyone from renting it until you got here. It's parked right outside the door warming up."
-Another time, I was delayed in MEM & going to miss their 5PM closing (it was a Sunday & turned out I got in after midnight). I couldn't find a taxi that would get me and also take me to work in the morning, so I had to call FSm to see if they could leave me keys with a CO employee or anything else before they left for the night. She made the trek back into the airport when my plane landed to give me a car!

Unfortunately, those days are gone; not just with me & FSM, but with that level of service from Avis, in general. I made sure to take care of "the ladies in FSM" in return. I always took in snacks/cookies/chocolate or small gift cards for Starbucks and wrote to Avis.com every single time. I'm not sure if they still do it, but back then, if you wrote a compliment with the employee's name, they got $25 from Avis!