To take tape off the back of posters when I take them off the wall, I have the poster laminated before posting. This makes the poster last much longer. I use only wide transparent polypropylene package tape to mount the poster. This type of tape sticks very well, even on painted cinder block walls. The tape should come off the poster easily at the end of the year. If not, I use my other trick of getting tape and labels off things: a hair dryer to warm the tape or label. I pull slowly and keep rewarming the next section.