By train
There is a train station in neighboring Conshohocken, at Washington and Harry. Regional Rail R6 will take you to the city center. The train ride takes about 30 min, and the fare is $5 if you buy tickets on board $4 if you buy in advance. Check SEPTA's website for timetables.
The station has a park-and-ride lot, but spaces may not be available to tourists.
There is also a train station in KoP, on Gulph Rd southeast of the mall. However, it would be a long and dangerous walk from the mall area, and there may or may not be short-term parking there.
By bus
While it may seem like the only way to see anything is to drive your own car, this turns out not to be the case.
SEPTA SouthEastern Pennsylvania Transport Authority runs buses and trains.
Bus 125 runs from KoP directly into the center of Philadelphia. You can catch it along Gulph Rd. You will arrive near the train station at 30th St and Market St; from there, you walk a few feet to the station itself and get on the eastbound subway towards the Old City and other attractions. As of 2008, the bus fare is $2 per trip. The buses run every 20 min during peak hours and every hour off-peak, during the weekdays, and the trip takes around 40 min.
While taking a bus may seem inconvenient, you will save time and hassle by not driving into Philadelphia yourself. Relax and let the bus driver fight the traffic. You will save even more time and money since you don't have to find a place to park your car in the city!
In the reverse direction, the last bus leaves Philly again at 30th and Market at about 10PM, on weekdays. Check the SEPTA website (http://www.septa.com) for specific information.
By car
Get in the car and drive. If it is for a day trip to the mall or just to go see what King of Prussia has to offer, the car is the best bet. Besides the traffic on the Pennsylvania Turnpike and Interstate 76, the car is quickest and most convenient way to get King of Prussia. If your coming from Center City, Philadelphia, get on I-676 Vine St Expressway, then go I-76 towards Valley Forge. Stay on Valley Forge for about 30 minutes. After proceeding on I-76 get off at the King of Prussia Exit. If you want to go to the King of Prussia mall follow the exit signs for Mall Boulevard. If your coming from the suburbs of Philadelphia, take the Pennsylvania Turnpike towards Harrisburg, then proceed to the Valley Forge Exit.