Find some way to charge the victim positively, before they leave the house/car and they would become a good conductor. Maybe have them walk along some big carpet, or somesuch, to give them a positive static charge. This would encourage a lightning strike of negative charge toward the victim.
Although there are positive lightning discharges, they account for only 5% of all lightning strikes. So giving the target a net positive charge would I think, give you the most chance of success. Also positive lightning discharges can be much more powerful and so more likely to be fatal, so if you want to keep your victim from a more likely death then you want them to get hit by a negative discharge.
The Positive and Negative Side of Lightning
How much charge you would need to give to the individual to give him an appreciably higher likelihood of getting hit, would depend on the weather conditions mostly I think. You'd want to put your target beneath the lowest point in the storm to maximise your chances of success and certain periods in the life of a storm are more likely to produce lightning than others.
How Lightning Develops Between The Cloud And The Ground