Take the 210 (Foothill Freeway) to Oak Grove. Turn north (right from the East, left from the West) and proceed to the guarded gate. See http://www.jpl.nasa.gov/about_JPL/maps.cfm for more details.
Here are some local bus routes:
- http://www.glendalebeeline.com/ (route 3)
There is a bus stop right outside the main gate.
From the main gate, the guards will direct you to the free parking.