Center City Philadelphia is a parking ripoff.
My usual option is to find a suburban station with plentiful nearby free street parking and take either commuter rail or a trolley in.
I've also parked at the airport in economy and took the airport line in for longer trips where I didn't want to leave my car on a strange street for a week or two.
Due to the terrible traffic near 30th street, the commuter rail is often a pretty good option, if you park near a station that is on the way to your final destination. (For example, if you're going to NJ, park near the Trenton or Atlantic City line.).
Google maps street view can be good for scouting out streets that allow for free overnight parking in the burbs.