I’ve had good experiences using the command poster strips. You can buy them at most stores. There are some other brands that have similar products, but the command ones are the only ones I’ve ever used.
I always used that “sticky tack” stuff when I was in college. It comes in strips, looks like rubber, but it moldable like chewing gum. Comes off pretty easily at the end of the semester.
Glue the poster to a piece of cardboard, then trim the cardboard around the poster, but leave a small border. Then you can use thumbtacks to pin the cardboard directly to the wall by using the border. You can even paint the border if you like.
You could tack it, which is cheaper then the frame. But the frame will leave the minimum amount of damage on the wall and poster, like @chels suggested.
@Seaofclouds is exactly right. Those little things are amazing. I use them almost exclusively for hanging posters and frames. Once applied, it’s easy to straighten the poster, and if you want to remove them from the wall, they come off really easily without damaging the paint.
Blu-tac or similar isn’t bad but it does leave greasy marks on the poster if left for a long time. You could protect the corrners by sticking a small bit of tape on the back.