ask them if they will be brave when people are offering their namaz-e-janaaza? If they are the "holy" type - tell them that not wearing seatbelt is akin to "suicide" and suicide is a sin - you go to hell for it.
for makeup freak ladies tell them that if they dont wear the seat belt - and they crash - their pretty face will be permanently disfigured after the crash and no amount of makeup or surgery can correct that - They will immediately start wearing it.
For the new "nakaam romeo stereotype" tell them that chicks dig this, vice versa for birds/lasses - and watch them immediately do it (albeit only infront of colleges or coffee shops)
For the "gym dweller" tell him that its the new super masculine fad - it will make your pecs and neck look amazing - sim,lar for ladies, except pecs etc - replace them with any proper feminine appendage name as you please.
different strokes for different folks.