You would have to "root" your phone (you need to look up how to do this if you want to do it)  and have a program call "apps2SD" that allows programs to be stored on the SD card.  By design from Sprint / HTC all apps are stored on the phones internal memory.  You do not want to get too low on internal memory or the phone will not work (not even for phoe calls).  For the time being you call go to menu> Settings> Applications> Manage Applications.  Wait for the list to populate then sort by size.  if you browse the net alot you can select Browser and clear the cache sometimes I have 3-4mb of memory that I clear up that way.  I don't mess with to many others because I do not know the performance issues if I clear the cache of other programs.  Maybe someone else can chime in on that part.