Świecko border crossing
Directly at the border there is a suitable place where cars drive relatively slowly and where it's quite easy to get a ride. There are some shops, currency exchange offices and Polish Customs. In summer there might be quite a lot of dust in this place - be careful if you have an allergy. Depending on the weather and season, there may also be mosquitos around.
Supposing you didn't hitchhike to Świecko, you can get there easily from the Słubice railway station. When you get out of the station, go left (southward) instead of right (as to the city of Słubice). You will soon see a lot of trucks, simply follow them and you will get to the checkpoint at the Polish-German border.
If your driver is going to Berlin and you want to hitchhike further, the border crossing is good place to get dropped off. Sometimes, most cars seem to go only to Berlin. It might even be better to get out at the last petrol station in Poland a few kilometers before, because most drivers stop there to for cheap fuel and cigarettes. Don't accept a lift to the next ("Biegener Hellen") service station in Germany (15–20 km), as very few people stop there.
It is also possible to get out here if you want to go to Frankfurt an der Oder. Still, from here to Frankfurt it is a tough walk of around one hour, and it will be difficult to catch a ride because traffic is fast and prostitutes are working here that might cause misunderstandings about your motives for stopping a car.
You can find a driver to Świecko quite easily along the A2 for example in Michendorf. From the border crossing you can easily catch rides to the bigger cities in Poland, but with some luck you could even catch a ride to Lithuania, Ukraine, Belarus. Used a sign is advised.
Since the construction of the highway was finished in late 2011, the border crossing looks a bit different. Cars and trucks pass now faster, but behind the border crossing is a turn after which you can hitch from the emergency line which is big enough to stop. However, n0id was sent away after 10min by the police and told to ask drivers who stop at the parking, because hitching there was to dangerous. Still, before he could even go back a truck did stop. User:Fverhart had a good experience on a Sunday late evening, and got a 151 km ride to the tollgates on the A2 before Poznan (PPO Goluski).
Many trucks that are going only to Poznan don't take the highway, but instead the old national road. If you want to go further, make sure that your driver is staying on the highway.