I normally let mine idle in the garage while I set up my loading ramps and keep it idling in the truck until after its tied down and all my other gear is loaded and I'm ready to leave.
When I get to the trail, I'll idle it in the truck while I get my riding clothes on, un-strap it and unload and wait for everyone else to get ready.
Biggest thing is to put it in neutral while it idles so the secondary clutch moves with the belt. If you have it in gear (not sure about park, I always just use neutral) the secondary clutch won't spin freely, causing the potential for your belt to rub a shiny spot on the clutch "walls" and the edges of the belt itself. It'll heat up pretty quick with all that friction, so if I'm ever stopped for more than a second or so, I shift to neutral.