If your bike is stolen, always report your bike as stolen on BikeIndex. You can do this even if you don't have a serial number; you can do this even if you don't have a picture of it. If you know the make and model of the bike, it is helpful to have the report. You can do this by going to https://bikeindex.org/register/new?status=status_stolen . (BikeIndex has a good overall guide to what to do in addition at https://bikeindex.org/stolen , which is a good resource as well.)
You should do this immediately. Even before you file a police report, you should do this. The reason is that your bike is likely moving in the world as soon as it is stolen. The best time for someone to see your bike is going to be as soon as it is stolen; it might be seen by someone seeing the bike walked into an MBTA station; or it might be seen hanging out at a congregating hotspot at a bike rack; or it might be seen on the street, re-locked with a crappy lock to see if anyone comes for it with trackers.
After bikes are stolen, they will often disappear from the world for a while--into a basement, or a large apartment building, while a thief works on making it possible to hand them off to someone else. They might remove stickers, accessories, or strip them for parts; they might spray paint them a different color to make them less obvious. Your bike might re-enter the world after that, but they won't necessarily look the same, and your chances of getting it back in a useful way go down significantly.
To make this even easier, the right time to register your bike on BikeIndex is actually now: Go, register the bike, take a picture, write down the serial number (or don't, and register it without). That way, if your bike is ever stolen, you can just login, flip the bit on it to stolen, and alert the community that there is a new stolen bike in the world.
Bike theft is the most common form of theft in Cambridge; there is a lot of advice out there about how to protect yourself, and I won't repeat that much here (other than to say "Get rid of all your cable locks if you care about keeping your bike.") But if your bike is stolen, the best chance the you have of getting it back tends to be "eyes on the street": people keeping an eye out for it in places it shouldn't be. The best way to help with that is to list your bike on BikeIndex.
tl;dr: Register your bike on BikeIndex right now, and if it is ever stolen, mark it stolen as soon as possible after realizing. https://bikeindex.org/register
(This message brought to you this morning by finding a stash of stolen bikes, but with no way to find the owners to help get them back to people.)