You need to taper the glass, I\'ll show you what I mean with an example I did for a 3dsmax6 Tutorial. It\'s animated, so I\'ll just render one frame. If you make the glass and the sand the exact same shape (clone glass to make sand, you will get a better result.
Here is my hourglass.
Edit: I just realised that Magerranger was referring to this very tutorial. Haha. Great minds think alike? Make sure you taper it in the center, and have the sand modelled only in one half, as there is only enough sand for one half at once, or else it would not work.
