You should never, ever use blu-tack to hang posters. I learned this the hard way after several years of sticking gobs of blu-tack to my bedroom walls to hang my posters. The damage done to the paint after removing them is quite bad:
It’s a strange double-edged sword. Use too little blu-tack to avoid damaging your wall, and your posters will most definitely fall off. Use too much and it will stick too tightly to the wall, which will cause potential damage months/years later. Or worse, the blu-tack might actually stick too hard to the poster itself and cause irreversible scars.
One of my oldest items, a 12-year-old Warcraft III: The Frozen Throne poster, was torn at the edges as I was trying to remove the blu-tack. Good thing I had planned to throw the poster away or I would have been very upset.
Fortunately I discovered an excellent alternative: Japanese washi tape. Apparently it’s made from Japanese rice, and it is highly reusable while remaining durable.
I’ve tested the tape on all of my posters, and after almost a month none of them have fallen off. I’ve actually repositioned a few of the posters, and it was effortlessly easy — the tape is easy to remove, won’t damage anything it was stuck to (just remove it slowly and carefully) — and sticks back easily after reapplying.
As you can see from the images, applying a layer on opposite sides of the poster is enough, although you can reinforce a third side, just in case, for heavier posters. Be sure to choose a colour that matches your wall — I went with ocean blue, while my sister picked purple (her posters not pictured).
There are many online stores that sell washi tape. I live in Singapore, and ordered from MT Masking Tape. One roll costs about SG$3.00, and there are several designs and colours to choose from. The more elaborate patterns can go up to as much as SG$8.00+. Do a little googling and find a store that can ship easily to you.
Give washi tape a try, and save your walls from blu-tack injury. Your posters deserve much better.