I occasionally receive "cold-call" applications from graduate or postdoctoral candidates interested in working in my research group. Most of the time, they are of no interest—it's just a "form letter"-type application. However, once in a while, I get an application from someone who I would consider giving an interview to—if I actually had an open position.
What I want to do is to let people know that I am interested in such candidates, even though I don't have an open job for them. Is there a way to express this interest, and to encourage them to apply again when a new position becomes available. Is there a good way to do this—most of the time, such emails sound very trite, and that's exactly what I'm trying to avoid.