At the upper-right of the main panel there's a drop-down menu where you choose the view for your playlist. There are sorting options there.
If you want to save the new order permanently, right-click the playlist in the
playlist explorer and select List->Update Play Order. This will make the current sort the new "manual order."
Also, if you use tracks view, you can click any of the headers to sort by that field and then do the List->Update Play Order thing to save it.