For getting on the A9 motorway you could either take the train towards Pegnitz and head for the petrol station there (i have just looked it up on the map and not yet tried it out, but it should be possible).
Straight from the city
For people trying their luck with a sign or thumb, you should head for the motorway exit Bayreuth Nord. Compared to the exit Bayreuth Süd, it is a lot easier for the cars to stop for you and there is even a petrol station close to the exit with the possibility to ask people directly. For getting there you take the Bus Nr. 1 towards Laineck. It leaves approximately every 20 minutes either from ZOH or the main train station. Get off at Königsbergstraße and head 100 meters towards the Autobahn. Take either the Esso Petrol station there or try your luck with the cars passing and stopping at the traffic lights. Take a sign, as there are several possibilities to go. Another option (if heading north) is after the big roundabout straight at the highway ramp. Cars are not going too fast (due to the roundabout), they can see you there is enough space to stop and you will not get into trouble as long as you stand before the sign marking the start of the highway. To get there take the same bus, get off at the train station in Laineck and head North-East through the industrial area until you see the roundabout.
On the ramp Bayreuth Süd there is also a major traffic light were you could flag down cars but no patrol station and very limited space for the cars to stop. Afterwards, straight before the ramp, there are no (legal) possibilities for the cars to stop.
Rest Place Wolfsbach
Another possibility is to get to the small resting area at the A9 in Wolfsbach by bus. From the bus-stop you pass the village towards the motorway and head slightly south towards a tunnel under the motorway. Just before the tunnel, there is a small road to the left passing a railroad bridge and going south along the motorway. Follow that road for about one kilometer. Right after passing a farm building (to the left) there is the resting place to the right. There is no petrol station here, but the people using the toilet will pick you up. There are rest places on both sides of the motorway.
Service Station in Pegnitz
The petrol station in Pegnitz is a good place for people with a “Semesterticket” or someone who is willing to take the train for a bit. You should have no problems getting lifts from here. Get the train to Pegnitz (30 min) and from here take buses 311 (->Nürnberg) ,386 (->Betzenstein), 391 (->Gößweinstein) and get off at "Neudorf West" (7min). From here head north through the commercial area for 1 km if going south or cross back under the motorway before heading north at its eastern side of you want to hitch North.
North and West (Kulmbach, Coburg, Bamberg, Schweinfurt)
From the city center you can walk the Hindenburgstraße until you cross the Nordring. After the crossing you'll find a busstop. From there many cars go in direction Kulmbach - and some also on the A70 to Bamberg or Schweinfurt (can be better with a sign to this direction), but at least Kulmbach should be easy.