If you're down there, don't hesitate to visit the overwhelming Old Town!
Take the mini bus #5 towards Trud. This one is driving out of town direction north. If you're coming more and more into a rural area, you'll recognize the motorway in front of you. Scream stop (spri in Bulgarian) to the bus driver to let you out near the ramp if you want to go east- or westwards. For going more north, wait until the bus enters the not-so-nice village of Trud. You'll find an old factory to your left. Get out of the mini bus when he leaves the main road. From there, walk some 100 meters north until end of town. There you'll find a petrol station. That's your spot for getting rides towards Karlovo or Hisarya. Another option towards Burgas, Varna
East towards Haskovo, Kapitan Andreevo
I didn't really take any bus, just walked. When you are in the centre, next to the main tourist information look for a Knyiaginia Maria Luiza street and follow it until you find yourself on the Tsarigratsko shose. There are plenty of hitchhiking spots, but I would recommend walking a little bit further to increase your safety against local gypsies and just catch more cars going out of the city. To find yourself at this place you need to cross the circle crossing and keep going straight on. Once you notice a bridge, cross the bridge and after the bridge there will be a hitching spot waiting for you.
Or you can also in front of the main railway station (Centralna Gara) take the bus 26 or 36 and take off at Tsarigradsko Shose. Walk a bit further till the beginning of the bridge where you can start thumbing.