How can I make prepasted wallpaper stick better?
Using wallpaper paste when hanging it will make it stick better.  

From the photo it looks like the paper is too short in the corner. 

To fix peeling paper use dilute white (PVA) glue.

Probably the you would have benefited from doing more preparation to wall surface too (sand it and size it). it looks very lumpy.

http://homeguides.sfgate.com/need-size-walls-hanging-wallpaper-77898.html