Go to Bostix. Bostix are located in two locations in Boston, one in Copley square and the other in Quincy Market. The two kiosks offer "day of" ticket sales for a number of events and theater shows at a discount of up to 50 percent. It is important to arrive early, as the tickets sell quickly. Bostix also offers museum discounts.
Join Stage Source to receive a list of upcoming shows. Stage source pools information about Boston's major theater organizations. Individual membership is fee-based, so decide if you are willing to pay to receive the future discounts. Members are also given weekly discounts and free tickets to local shows. Discounts are only available to stage source members.
Purchase "rush" tickets. Many theaters in Boston offer student rush tickets, however you must arrive several hours prior to the start of the performance in order to receive the discounted tickets. The tickets usually cost $25 and offer good seating options.
Contact the venue to inquire about large group discounts. A theater company may offer discounts to fill their seats for a particular performance, so call in advance and provide the names of each person in your party.