Well, as with all dream control - its about belief and expectation. And confidence.
The same goes for summoning people.

As a result though, its easier to believe in some things than others.
For example - if you summon the people coming down the road behind you, its a whole lot easier to believe in than imagining them 'star trek' beam up in front of you
The beaming technique might be more fun, but takes a bit more confidence to pull it off.

Dream control is all about learning how to expect things to happen - confidence...