Plastic wallpaper smoother or other flat instrument Even an old credit card can work in a pinch! Warning: make sure the applicator is plastic, NOT metal, or you could tear your poster
Artists gel medium in matte or gloss I prefer the Golden brand for it's quality and consistency. I use this medium for my abstract art so it's always on hand!
Notes
If you purchase a canvas that's the exact size of your poster, this will save you the hassle of cutting the poster down to size. You can also chose a canvas slightly larger than your poster to create a small frame around the edge.