I've found its a lot of trail and error mate, worth noting that paint FX trees when converted (if your going to render in mental ray) are very high poly and will crank up render times a lot.
Suppose the best thing to do is play with the settings (have a look in the help files for the different attributes for what you want) and the njust do lots of test renders, I madea spookey tree from a tutorial that looked ok heres a render, all done in maya, it is animated but you get the idea.
edit - If you want e-mail me and ill send you the file
"No pressure, no diamonds" Thomas Carlyle