Parking for visitors?

[mindstalk.livejournal.com]

I know parking's hard around here, but walking around the Porter Square area, it seemed like you can't even address the problem with money. Two hour parking everywhere; free in Porter Square but only for customers (enforced how?); longer in the Exchange but paid and customers only; permit only on Cambridge side streets. Is there nowhere to direct someone to park for a day visit, let alone night?