This is a spot where I highly recommend experimenting. I used to order the mushroom and caramelized cheesesteak every time. Once I decided to order a regular cheesesteak which comes with lettuce, tomatoes, and mayo. I forgot to specify that I didn't want mayo, but it turned out pretty good; so much so that a cheesesteak sandwich from Uncle Sam's is one of only two sandwiches in the world that I will accept mayo on.
The other tip that I can offer is not to pay with a debit or credit card. Basically, you are charged a transaction fee from Sam's in addition to any fees that your bank may charge you.