You can buy really cheap poster frames. I bought some at an arts and crafts supply store, but I think they might have them at Walmart as well.

I know some ppl frame them

sticky tack i think

you could try sticky tack, try getting a color that is close to your wall color, so it wont stain. ( if your wall is white get a white sticky tack color)

I had this problem when I first got posters too.. But I ended up using tacks because I didn't wanna ruin the posters worse with tape and if you really care about your wall for when after you take the posters down or something Home Depot has like wall fillers??? Like you can fill the holes with it and itll look like there wasnt a hole! Idk if this was helpful at all but yeeee this was my reasoning LOL

I find that cellotape works to hold the poster on the wall, but for certain posters it ruins them because of the paper TT^TT. I have resulted in using blue tack, but I'm not concerned about the walls and I usually don't have a problem with it coming off the wall anyway.

try buying either scotch tape (the ones that aren't like see through)there very gently and won't damage your wall or the posters (if your posters has a glazey feel then the scotch tape comes off easily) Do not buy masking tape thats those big round clear tapes, they are horrible and very damaging, you can also buy 2 sided tape, the roll is completely white and you just peel it off on both sides, both sides are sticky. The 2 sided tape and scotch tape might not last a long time but it's better than masking tape. With my posters (I have over 30 of them) they slowly become not sticky but I just change the tape