The "Chinatown Buses" are the best thing to happen to East Coast college students, and other people low on funds, since Ramen Noodles. Fung Wah bus company only shuttles to Boston (other services to to D.C. and Philadelphia), and it's rates are comparable to the other services ($20 round trip). Buses leave from Chinatown, NY every hour and are every bit as nice, if not nicer than Greyhound. I've taken it several times, and once took another bus company to DC (the other bus was awful and smelly), and the Fung Wah bus was always clean, on time and made great time. And I don't remember the bathrooms being terrible, so as far as bus bathrooms go they must have been okay.
Evening buses fill up fast on the weekend. You can show up to buy tickets (usually sold out of a storefront at the departure address), but I recommend reserving them through the website ahead of time.