I just discovered this enormously handy website that finds parking garages in the city for you!  You simply input your destination and it brings up a list all of the surrounding parking garages for you.  Not only does it find them, it tells you exactly what they cost.  I mean, you can actually input the dates & times that you are planning on parking there and it will calculate the rates for all of the garages "for that timeframe" for you.  (i.e. I'm looking to park on Sunday from 3-5:30pm - how much will that cost?)  That way you can decide by price OR by proximity to your destination. 

I'm so happy I found this.  Now I can actually figure out where to park in advance without paying a fortune!

Right now the site works for the cities of Boston, New York, Washington, DC & Philly.