There is a radio program out there on the 'net somewhere (I forget where I got it from) that is an interview with LaBerge, and he talks about that specific issue.

In it he said that if you want someone to appear, you need to do it in a way that they would normally appear in waking life. So instead of just trying to get the person to appear out of thin air, try instead to have them show up from a logical waking context. So, if you are having a dream where you are at school and this person you want to appear might be in a certain classroom in waking life, go into the classroom and before/as you open the door tell yourself that that person will be in that room when you open the door.

I have never experienced this first hand, but that was the gist of what he said.