Depends on the bike. Some bikes have a good passenger peg location and the helmet hangs there just fine.
Most times, I just set it on the pavement. I do this because I've never found a motorcycle seat that could contain the suicidal tendencies of my helmets - they always jump off at the most inopportune moment.
Never hang your helmet on a passenger peg while at a bar - with the opening "up". There's always that one guy........
'15 FLSTC, '14 Versys,
'13 FJR1300, '12 DR650
'10 GL1800,'12 VN900B,