There are a couple of tricks for people parking in Stadium Lots to get out reasonably quickly if you stay for the whole game, but I'd hate to give them away on a public forum. A lot of people, including myself, have had to go through a load of trial and error to get it right. Plus, the exterior lot on Jamestown that you can pay for on game day was pretty convenient for me when I was a single game ticket buyer.
If you and your party are willing to walk, I know a lot of people park near the CalTrain Bayshore Station. It is about 2 miles with an incline to get over the freeway, though. There might be a shuttle, but I am not sure.
There is a Muni shuttle at the Balboa Park Bart station.