Heading south or northEdit
If you want to go south or towards Lueneburg you should start at Raststaette Stillhorn. It is easy to get there. Just take S3 or S31 to Wilhelmsdorf and then a bus to Kirchdorf-Süd. You get off at the last stop. Just follow the road and then you will hear the highway. After crossing a little bridge you're already on the parking lot of the Raststaette. Walking distance is about 3 minutes.
If you're planing to hitchhike towards Berlin on the A24 it is best to get to the big roundabout in the southeast of the city. You can't reach it directly by U-Bahn or S-Bahn, so you have to walk. The easiest way is probably from the S1, S11 and U1 stop Wandsbeker Chaussee. From there you have just to walk straight on Hammer Straße. When you reach the roundabout it is difficult to get a ride standing in the middle of it, as the cars have no possibility to stop. But on one side, in Sievekingsalle, there is a busstop. This works fine.