Vaseline (petroleum jelly) is a good lubricant, but it will eventually destroy your cube.
Silicone spray is also good, and will not destroy your cube.
You can also get Jigaloo, which is also a pretty good lubricant.
You can get silicone spray or Jigaloo at Wal-Mart or you local hardware store, if you don't have it.
Shouldn't this question be in the Hardware sub forum of this website?