We always try to fly out of Newark but for our next trip we are going out of Philly. When flying out of newark we always use Vista airport parking (I'll find the link), they are great! The price is about $9 per day and they take your things from your car, park it, and bring you to your terminal. The parking lot is right at the Howard Johnson and to my understanding the company is run and manned 24 hours per day by retired and off-duty police officers. The lot is all fenced in and we have never had anything but a wonderful experience with Vista. When you get back to the airport with your bags just call them and they will be there waiting (we have never waited for a van) - when you get to the lot your car will be waiting.
I don't think I would ever park my car at the actual airport, its expensive and long term parking is very very far away. They have a shuttle type thing for airport long term parking but I've never heard of anyone that has had a good experience with it.
Newark airport is a pretty good airport since the renovations, especially the C terminal. Best of wishes!