[iTunes] Smart Playlist to pull different music from a playlist

[i.am.list.boy at gmail.com]

Question I have:

I have created a Smart Playlist to randomly pull 300 MB of music from  
a playlist which holds 5 GB of songs.

I want this playlist to load my 1st Gen Nano with a "fresh" set of  
songs each time I sync it.

But I'm missing something, as I can't seem to get it to get a new  
batch of randomly selected music when I sync it the second time.

Anyone else in my boat?

Russell