The poster's were EASY. I ended up getting 100 every time.
My strategy: Run straight to the wall ahead of you, slap on the poster, head left and keep slapping on posters. Go towards where the train winds around, there are THREE slots on one board for posters. Do that. Keep going around where the train is, there are THREE more slots for posters on the building that the train goes around. From there, you can see some more slots for posters straight up ahead. Head up, slap on a poster, turn left, there is another one. Slap on a poster, and turn back around. Jump on the building, slap a poster on both sides of the wall, hug the wall and gradually jump down, slapping posters agains the buildings. Jump all the way off and towards the opposite wall there are 3 more slots for posters. You're done.
I did this about 20 times altogether, just for extra easy munny, so yes, I have it memorized.
I did this all in a span of 1 minute and 2 seconds.