Lots of good advice so far but practice practice pratice. I'm like a few of the guys on the site here, I tried for a few years and then one day it just happened.

ALl the talk about where the call should be (in your mouth) and using the toungue versus fluttering lips versus garggle etc. is important BUT have the right TYPE of call to begin with is also quite helpful. Now I'm not talking any particular call in general but there are certainly styles that allow you to purr better than others (I think anyone would agree with that one).

I get my best purring out of either a ghost cut or a center V-cut with 3-4 reeeds and that last reed being rather thin.

My best purring calls to date;

Woodhaven's Ghost cut (they change the name but I think it used to be called Ghost Cutter)
TalkingStick Game Calls (either thier Blackbeard four Reed or the Orange Frame with Green reed call (forget what the name is)

Just remember, the caveat with mouth calls, what works for me might not work for you.

Good luck and keep on practicing!