Fun fact: you're not allowed to put signs within 150 feet of the entrance to a polling location. The same applies to stickers ("pasters"), petition workers, sign holders, etc. Basically: nothin' except voters, observers, and election workers within 150 feet of the entrance, or inside the building of a polling location. From an admittedly small sampling, nobody seems to be heeding that rule, as at least one polling station I passed was festooned with signage well within that 150 foot zone (Google Earth and its ruler function are your friend.) Voting? You can possess, but not display, any campaign materials. This almost certainly includes stickers, buttons, t-shirts, and so on. Personal notes are A-OK.
Virtually every step of the process must be done in public view, starting with showing the ballot box to be empty and then locking it. The box cannot be removed from public view or opened, except to press down ballots to make room for more. Polls must be open for voting for at least 13 hours, and anyone in line when the polls close must be allowed to vote. Further, the polling station must remain open to allow the public the oppertunity to watch votes be tallied and sealed. If you fudge a ballot, you're allowed 2 more, and the "spoiled" ballots must not be inspected, but must be tracked/tallied, and sealed just like legitimate votes. Lastly, leave the smokes and beer at home, and you gotta fly solo in the booth (no hot democracy-themed fantasies, sorry), save for someone assisting you in marking your ballot (Well! Maybe not? Better make it quick, you two only have 5 minutes if anyone is waiting for a booth.)
State Secretary's summary, relevant chapter of the MGLs.
Problems? Call the Secretary of State: 617-727-2828 or 1-800-462-8683.