Split a membership
The membership fees at some warehouse clubs can be pretty steep, especially for penny-pinchers. Going in with a friend for a membership is smart, and will save you both money. After signing up, put your friend's name down for the other card and just split the cost.
If that friend happens to be your roommate, even better! You can buy in bulk shared household items, like dishwashing liquid and paper towels, and split the cost while splitting the membership fee. Every warehouse club has different rules regarding memberships, though, so be sure to check the club's membership regulations.