Zürich is a large city in eastern Switzerland.
The most direct option is to reach a petrol station on the motorway by train and then foot. Take the S12 to Kilwangen-Spreitenbach. The stop is near the service station. One can also use this petrol station to go east and south of Zürich by using the opposite side to the Bern/Basel exit.
The bus station Sportweg makes a great hitchhiking spot. It's located right at the motorway entrance, but there's a traffic light, so cars are not too fast. You can either go there directly by bus, or take a tram to Hardturm from where it's five minutes walking.
There are also many S-Bahn trains stopping at the train station Hardbrücke. (Or take bus #33 and #72, whose first stop after Hardbrücke is Pfingstweidstrasse or bus #54 to Sportweg). When you take a train, walk up onto the bridge on the side closer to the city/main station, then turn right and go down the pedestrians ramp, then straight until you reach the main road Pfingstweidstrasse. Turn left, cross the street. You can already start hitching here, there's enough space and cars, or you can walk closer towards the motorway. Or walk and hitch together. It's an excellent street. (update): As of June 2009, 'Pfingstweidstrasse' is all under construction and thus nearly impossible to get a ride from.
South towards Italy
Follow signs in the city for motorway 4 to Luzern. You will reach West st. Just past the intersection of West and Werd, there is a driveway in front of an apartment building where cars can pull over just before the right turn onto the onramp of motorway 4.
Take S-Bahn S7 to Kemptthal (~25 minutes from the main station), then walk up the hill in the direction from where the train came from. After a 5 min walk, you should see a grassy hill on your right hand side. Walk to the top of the grassy hill and you will find a BP petrol station, a restaurant and a small COOP supermarket.