How to show interest in a candidate when no positions are available? 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.
This answer is about faculty candidates, not graduate student or postdoc candidates.  (I misunderstood the original question.)

First, be honest with the candidate, both about your interest and about your ability to hire.  Yes, we are very interested.  No, we don't have any regular faculty slots this year.  Yes, we would love to fly you out, have you give a talk, meet with some of our faculty and students, maybe have a chat with the dean, and see where things go from there.  Make no promises you can't actually keep.

Second, a lack of open slots does not mean it is impossible to hire.  Your college or campus may have finds set aside for "excellence" hires, or for cross-departmental research initiatives (like "clean water" or "computational science"), or for dual-career families.  Some other overlapping department may be willing to give you half a slot for a joint appointment.  Another faculty member in your department or college may have just unexpectedly retired, resigned, moved to administration, failed to get tenure, or died.  Your dean may be impressed enough (or encouraged enough by other senior faculty) to give you an extra slot just for that person next year.

Creating a slot takes a lot of legwork and a lot of political capital—you may burn a future slot even if the candidate doesn't come.  So they'd better really be special, and you'd better know that they'll come if a position is actually offered.

Yes, I have seen this work.