Słubice is a little town at the German/Polish border. It's not easy to hitch out of this town as the motorway is some kilometers south of the city and only reachable after a long walk. An option might be to thumb on the local road and try to find somebody that can take you to the A2.
For further information about how to hitchhike from a service station on the motorway please check the article of the German side of the border town, Frankfurt/Oder.
Walk along the the road until the end of town. This road, national road 29, is the only route leaving the town to the south and goes straight to Wrocław. Several kilometers south of the city, there is an intersection with the highway A2 which goes east to Poznan and Warsaw. There are a few gas stations along the road where you can ask for a ride to or onto the highway. One about one kilometre after the intersection in direction of Warsaw is service station on the highway.
West towards Germany
Instead of hitchhiking from Słubice, you can walk over the bridge to Frankfurt (Oder) and take a bus or tram to the German motorway.
It is also possible to go to the parking place of the cigarette shop in ulica Kościuszki. You find it close to the roundabout opposite the Collegium Polonicum behind the shopping centre. A lot of Germans from Berlin and other places come here to buy cigarettes. You can ask them to get a lift, look out for their licences plates and if you approach them before they enter the shop you can point out that with one more person in their car they are allowed to legally import four extra cartons of cigarettes (yes, the number is correct, there is a particular agreement between Germany and Poland about the legal amount of cigarettes).