Turku (Swedish Åbo) is a coastal city in Southwestern Finland. It has an active harbour, with both commercial and cruise ships going to and from major cities in the region. From Turku, there are highways leading to all different directions throughout Finland, so make sure you know which highway is going your way, and always use a sign.
Going to Helsinki
From Stockholm, taking a cruise to Turku, rather than Helsinki, is comparatively less expensive, and you can always hitchhike the difference. The Viking Line cruise ships have free tourist maps of Turku at their information desks onboard. Be warned that if you take a daytime cruise (which is about half the price), you will arrive in Turku at night. In the Fall, Winter, and early Spring, that means no daylight for hitchhiking, and possibly extremely cold temperatures, as well. Plus, there might not be much traffic on the icy (or slushy) roadways, since Turku is a rather small city.
The highway entrance to Helsinki (and a few other places) begins just after the hospital, from the right lane. There is a spot where cars slow to turn onto the on-ramp, but can also stop & pull over. This highway (1), being a main one, has a lot of exits to other highways going to all different directions, but there are a few gas stations / rest stops in between Turku and Helsinki, so it shouldn't be too hard to find a warm spot indoors in between rides. However, since the distance between Turku and Helsinki is not too much, it should also be possible to get a single ride there.
You can also start from the very beginning of the highway number 1/the end of the street called "Helsinginkatu". This is just next to both Student village (Ylioppilaskylä) and the University of Turku. Coming from the direction of the harbour and the bus station on "Helsinginkatu", after the bridge take the right side of the street. After the traffic lights there's a bus stop where you can start hitching. A sign is a good idea since there's a lot of cars driving to suburbs of Turku.
Going to Tampere
Buses 1 or 18 go a short distance north of the centre to a crossroads in a neighbourhood called Raunistola. The point at which the highway to Tampere begins is signposted several hundred metres south of the crossroads, but unfortunately not at the crossroads itself. Look for a small sign reading "[HW] 9", a street sign reading "Tampereen valtatie", and an R-Kioski and a small Hesburger side by side. One can hitch from the north end of the parking lot outside of the Hesburger, сars will probably stop in the bus stop next to the parking lot. Use a sign due to the large amount of suburban traffic.