Question: There's a certain kind of people who like to hoard whatever limited resources they get "just in case"; they usually end up never using them, always waiting for bigger emergency. On the other hand, there are game mechanics based on spending some limited resourse: like bennies in Savage Worlds, fortune points in Warhammer, etc. You're supposed to spend them liberally, getting some in-game benefits. 

The "hoarders" never spend these, robbing themselves of part of enjoyment. Simply explaining that "those point are meant to be spent" doesn't always work: players seem to understand that rationally, but some force of habit still prevents them from actually using their resources. I think this happens more often in videogames (I myself am guilty of hoarding all goodies till the end boss, and then never needing them), but I've seen this kind of behaviour in tabletop games too. 

Of course, some players do this because they just like hoarding, or because they like the additional challenge, but some do notice that they're "doing it wrong."

Is there maybe some sort of trick to encourage resource spending? 

**Clarification**
I'm mostly interested in "mechanical" resources like fate points and bennies. If you hoard something material, like potions, or money, at least you (probably) have more money as a result. When you hoard bennies, you essentially forfeit your chance to *do* something that matters. This is probably not what players want.

Answer: 
I don't think you should think of this as a "trick". I think the players are being rational. They are doing all right without spending the resources, and they have no idea what is coming up, that may require them. 

So just make it rational to spend them. Have lasting rewards that make spending them worthwhile even when there is some risk of running out of the resource and not having it when they really do need it, or, I suppose, have them suffer permanent adverse consequences for *not* spending them.
Are you trying to change your players'/characters' personalities? Maybe! (Of course since it's an RPG that could be fun... read on)

Your question is asking about fungible resources, has no uniquenesss value, but *stores* value, and can be *spent* to gain some benefit... hey, sounds a lot like the economic definition of money, don't you think?

When it comes to money you can lump people into two types of behavior: spenders, (living more or less paycheck to paycheck) and savers/investors. You have just identified some of your players as savers.

Now since it's an RPG, maybe you should encourage players who tend to be savers, to imagine their character as being a little different, act like a wild spender for a change! When phrased that way it could be a liberating experience. 

Just try not to let it come out as 'Come on, you guys never spend anything!' as your player could just take it as personal criticism.