Prepasted wallpaper won't stick?

We bought a wallpaper border for our boys' room. It was the kind that already had adhesive on it. We had a very hard time getting it to stay on the wall but someone finally told us about something you paint on the wall to help it stay. It helped...a little. We finally got the border up all but one little peice that refused to stay. Now its all back down and I was wondering if anyone knew how to get it to stay. I will need to buy more because all we bought was ruined. I don't want to waste my money on something that will have to be replaced every 6 months or so. Any suggestions?