Take Buses #31, #12, #8 or #78 to the bus station called KTEL. From there take bus #80 or #80A to the village called ''Malgara''. You can go down on the station "''25 Martiou''" which is the closest to the tolls (in Greek ''ikostis pemptis martiou'' if you want to ask the driver). There you can ask and find the toll station (in Greek ''diodia'') easily. You'll have to walk about 2 km to find it, but then there is a possibility to get a ride even straight to Athens. Before you enter the motorway where the toll station is, you will have to go through some fields. If it's night be careful, just before the motorway there is a small ridge that is a bit steep and there is water flowing down, you need to find your way around it. If it's day, from the fields you can see a small canteen on the motorway, head to that direction, you can bypass the ridge there very easily and also you don't need to jump the fence (easy to jump though) because there is an entrance/exit for the canteen. From the canteen you will see the tolls, they are less than 50 m away.
For those not wanting to take a bus or want to hitch closer to the city (and aren't scared to walk), go to the the A1 motorway going towards Athens. It runs right beside the city centre. Starting from near the train station after walking 2kms you will arrive at the international bus station. From there it's a 6km walk along the motorway to where the E90 highway joins on to the A1 road to Athens. This is a good spot due to the high amount of Turkish trucks coming straight from the border and bypassing the city as well as any traffic coming from the north. Stand at the end of the onramp where there is a shoulder for traffic to pull over and you can get a ride to Larissa or even Athens.