If you have some flexibility with your end-of-day working hours, make use of the 24hr unlimited pass!

(Ex: One night I'll take the 6:27pm from Back Bay, the next morning I'll take whatever, and that evening I'll take the 5:45pm, which is within 24hrs. You have to buy these unlimited passes in 5-packs, but it's ~$24 per pass and I get 3 rides out of it, so cheaper than the $12.50 x 3 = $37.50)