the convention supposedly has magic vouchers which take the sting out of parking in the commons garage, it's basically a single discount on your ticket. the way to work it though, is that you have to MOVE your car in the early (8-9am? ish) and then put it back, to be able to use more than one voucher. it doesn't become too pricey that way. the garage isn't too far a walk either (do the drop off first, buy the driver who has to walk back a nice dessert at finale :>)

the previously mentioned alewife method is the easiest and cheapest method i know, short of having a friend with a parking space you can use in the city. so, you could do a big drop off, and then shoot the car over to alewife, and come back on the T. more "annoying" but more peace of mind generally.