Are you currently in MA? I had a hard time last year as well when I was trying to line up a place from out of state. I got to Cambridge on August 17 and found a place within 3 days. It was really hard to find a place even when I was in town. And it was VERY stressful omg. The place I ended up signing was way out of my original price range. You have to be extremely diligent at checking CL, sending emails, and making calls. Try to expand your search criteria and cast a wide net, and only look at ads that were posted within the last 24 hours. Anything posted before that has already been signed 99% of the time.
Also call a bunch of agents that you see on CL and tell them that you're a motivated renter for 9/1. Most of them won't have anything but there's always a chance that they will come across something.
Be ready to sign. The place I rented was listed on CL at 11:00 am, I saw it at 12:00 and committed to it, because another person was coming to see it at 12:30. It's an aggressive market. Good luck!
Thanks for the info! I'm still in London at the moment but will be in Boston by next Monday. Hopefully I'll be able to figure something out when I'm there. If not then my backup plan it just to sign a sublet/short term and keep looking during term time.